Method and device for transmitting and receiving synchronization signal of device-to-device communication terminal in wireless communication system

ABSTRACT

One embodiment of the present invention relates to a method for receiving a sidelink synchronization signal (SLSS) by a terminal in a wireless communication system, comprising the steps of: receiving a physical sidelink broadcast channel (PSBCH); determining which of global navigation satellite systems (GNSS) or an eNB is to be a synchronization source according to priority information included in the PSBCH; and receiving an SLSS associated with the determined synchronization source, wherein the terminal determines that the priority information is valid only when a public land mobile network (PLMN) associated with the priority information matches a PLMN to which the terminal belongs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/799,058, filed on Feb. 24, 2020, which is a continuation of U.S.application Ser. No. 16/064,730, filed on Dec. 10, 2018, now U.S. Pat.No. 10,575,269, which is a National Stage application under 35 U.S.C. §371 of International Application No. PCT/KR2017/010726, filed on Sep.27, 2017, which claims the benefit of U.S. Provisional Application No.62/458,590, filed on Feb. 14, 2017, U.S. Provisional Application No.62/458,562, filed on Feb. 13, 2017, U.S. Provisional Application No.62/426,219, filed on Nov. 23, 2016, U.S. Provisional Application No.62/407,513, filed on Oct. 12, 2016, U.S. Provisional Application No.62/403,050, filed on Sep. 30, 2016, and U.S. Provisional Application No.62/400,623, filed on Sep. 27, 2016. The disclosures of the priorapplications are incorporated by reference in their entirety.

TECHNICAL FIELD

The following description relates to a wireless communication systemand, more particularly, to a method and apparatus for transmitting andreceiving a synchronization signal in the case where a satellite signalis available for synchronization.

BACKGROUND

Wireless communication systems have been widely deployed to providevarious types of communication services such as voice or data. Ingeneral, a wireless communication system is a multiple access systemthat supports communication of multiple users by sharing availablesystem resources (a bandwidth, transmission power, etc.) among them. Forexample, multiple access systems include a code division multiple access(CDMA) system, a frequency division multiple access (FDMA) system, atime division multiple access (TDMA) system, an orthogonal frequencydivision multiple access (OFDMA) system, a single carrier frequencydivision multiple access (SC-FDMA) system, and a multi-carrier frequencydivision multiple access (MC-FDMA) system.

Device-to-device (D2D) communication is a communication scheme in whicha direct link is established between user equipments (UEs) and the UEsexchange voice and data directly without an evolved Node B (eNB). D2Dcommunication may cover UE-to-UE communication and peer-to-peercommunication. In addition, D2D communication may be applied tomachine-to-machine (M2M) communication and machine type communication(MTC).

D2D communication is under consideration as a solution to the overheadof an eNB caused by rapidly increasing data traffic. For example, sincedevices exchange data directly with each other without an eNB by D2Dcommunication, compared to legacy wireless communication, networkoverhead may be reduced. Further, it is expected that the introductionof D2D communication will reduce procedures of an eNB, reduce the powerconsumption of devices participating in D2D communication, increase datatransmission rates, increase the accommodation capability of a network,distribute load, and extend cell coverage.

At present, vehicle-to-everything (V2X) communication in conjunctionwith D2D communication is under consideration. In concept, V2Xcommunication covers vehicle-to-vehicle (V2V) communication,vehicle-to-pedestrian (V2P) communication for communication between avehicle and a different kind of terminal, and vehicle-to-infrastructure(V2I) communication for communication between a vehicle and a roadsideunit (RSU).

SUMMARY

An aspect of the present disclosure is to provide a method for selectinga synchronization source by assigning a priority level in associationwith a PLMN, and then receiving a synchronization signal, in the casewhere a satellite signal is available for synchronization.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present disclosure are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present disclosure could achieve will be more clearlyunderstood from the following detailed description.

In an aspect of the present disclosure, a method for receiving asidelink synchronization signal (SLSS) by a user equipment (UE) in awireless communication system includes receiving a physical sidelinkbroadcast channel (PSBCH), determining which one between a globalnavigation satellite system (GNSS) and an eNB is to be used as asynchronization source, according to priority information included inthe PSBCH, and receiving an SLSS related to the determinedsynchronization source. Only if a public land mobile network (PLMN)related to the priority information is identical to a PLMN to which theUE belongs, the UE determines that the priority information is valid.

In another aspect of the present disclosure, a user equipment (UE) forreceiving a sidelink synchronization signal (SLSS) in a wirelesscommunication system includes a transmitter, a receiver, and aprocessor. The processor is configured to receive a physical sidelinkbroadcast channel (PSBCH) through the receiver, to determine which onebetween a global navigation satellite system (GNSS) and an eNB is to beused as a synchronization source, according to priority informationincluded in the PSBCH, to receive an SLSS related to the determinedsynchronization source, and to determine that the priority informationis valid, only if a public land mobile network (PLMN) related to thepriority information is identical to a PLMN to which the UE belongs.

The UE may always select an SLSS ID from a range of 170 to 335irrespective of synchronization resources configured by the network orpreconfigured.

The PSBCH may include a PLMN ID.

If the UE receives an SLSS for which the GNSS is a synchronizationsource, the UE may always transmit an SLSS.

If the UE receives an SLSS for which the GNSS is a synchronizationsource, the UE may transmit an SLSS for a predetermined time even thoughthe UE loses GNSS reception.

A synchronization source may be configured per a resource pool.

If the synchronization source is the GNSS, a bitmap of a resource poolrelated to the GNSS may be based on D2D frame number (DFN) 0.

According to the present disclosure, a timing difference can beprevented, which might otherwise be produced if the priority levels ofsynchronization sources are different according to PLMNs.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present disclosure are not limited to whathas been particularly described hereinabove and other advantages of thepresent disclosure will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this application, illustrate embodiments of the disclosure andtogether with the description serve to explain the principle of thedisclosure. In the drawings:

FIGS. 1A and 1B are views illustrating the structure of a radio frame;

FIG. 2 is a view illustrating a resource grid for the duration of adownlink (DL) slot;

FIG. 3 is a view illustrating the structure of a DL subframe;

FIG. 4 is a view illustrating the structure of an uplink (UL) subframe;

FIGS. 5A and 5B are views illustrating the configuration of amulti-antenna wireless communication system;

FIG. 6 is a view illustrating subframes each carrying a device-to-devicesynchronization signal (D2DSS);

FIG. 7 is a view illustrating relay of a D2D signal;

FIGS. 8A and 8B are views illustrating an exemplary D2D resource poolfor D2D communication;

FIG. 9 is a view referred to for describing a scheduling assignment (SA)period;

FIG. 10 is a view illustrating an exemplary situation in which a globalnavigation satellite system (GNSS) may be used as a synchronizationsource;

FIG. 11 is a view illustrating PLMNs and exemplary selection of asynchronization source; and

FIG. 12 is a block diagram of a transmission apparatus and a receptionapparatus.

DETAILED DESCRIPTION

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions or features ofany one embodiment may be included in another embodiment and may bereplaced with corresponding constructions or features of anotherembodiment.

In the embodiments of the present invention, a description is made,centering on a data transmission and reception relationship between abase station (BS) and a user equipment (UE). The BS is a terminal nodeof a network, which communicates directly with a UE. In some cases, aspecific operation described as performed by the BS may be performed byan upper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a UE may be performed by the BS or network nodesother than the BS. The term ‘BS’ may be replaced with the term ‘fixedstation’, ‘Node B’, ‘evolved Node B (eNode B or eNB)’, ‘Access Point(AP)’, etc. The term ‘relay’ may be replaced with the term ‘relay node(RN)’ or ‘relay station (RS)’. The term ‘terminal’ may be replaced withthe term ‘UE’, ‘mobile station (MS)’, ‘mobile subscriber station (MSS)’,‘subscriber station (SS)’, etc.

The term “cell”, as used herein, may be applied to transmission andreception points such as a base station (eNB), sector, remote radio head(RRH) and relay, and may also be extensively used by a specifictransmission/reception point to distinguish between component carriers.

Specific terms used for the embodiments of the present invention areprovided to help the understanding of the present invention. Thesespecific terms may be replaced with other terms within the scope andspirit of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems,Institute of Electrical and Electronics Engineers (IEEE) 802, 3rdGeneration Partnership Project (3GPP), 3GPP long term evolution (3GPPLTE), LTE-advanced (LTE-A), and 3GPP2. Steps or parts that are notdescribed to clarify the technical features of the present invention canbe supported by those documents. Further, all terms as set forth hereincan be explained by the standard documents.

Techniques described herein can be used in various wireless accesssystems such as code division multiple access (CDMA), frequency divisionmultiple access (FDMA), time division multiple access (TDMA), orthogonalfrequency division multiple access (OFDMA), single carrier-frequencydivision multiple access (SC-FDMA), etc. CDMA may be implemented as aradio technology such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented as a radio technology such as globalsystem for mobile communications (GSM)/general packet radio service(GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may beimplemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, evolved-UTRA (E-UTRA) etc. UTRA is a partof universal mobile telecommunications system (UMTS). 3GPP LTE is a partof Evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA fordownlink and SC-FDMA for uplink. LTE-A is an evolution of 3GPP LTE.WiMAX can be described by the IEEE 802.16e standard (wirelessmetropolitan area network (WirelessMAN)-OFDMA Reference System) and theIEEE 802.16m standard (WirelessMAN-OFDMA Advanced System). For clarity,this application focuses on the 3GPP LTE and LTE-A systems. However, thetechnical features of the present invention are not limited thereto.

LTE/LTE-A Resource Structure/Channel

With reference to FIGS. 1A and 1B, the structure of a radio frame willbe described below.

In a cellular orthogonal frequency division multiplexing (OFDM) wirelessPacket communication system, uplink and/or downlink data Packets aretransmitted in subframes. One subframe is defined as a predeterminedtime period including a plurality of OFDM symbols. The 3GPP LTE standardsupports a type-1 radio frame structure applicable to frequency divisionduplex (FDD) and a type-2 radio frame structure applicable to timedivision duplex (TDD).

FIG. 1A illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as a transmission time interval (TTI). Forexample, one subframe may be lms in duration and one slot may be 0.5 msin duration. A slot includes a plurality of OFDM symbols in the timedomain and a plurality of resource blocks (RBs) in the frequency domain.Because the 3GPP LTE system adopts OFDMA for downlink, an OFDM symbolrepresents one symbol period. An OFDM symbol may be referred to as anSC-FDMA symbol or symbol period. An RB is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols in one slot may vary depending on a cyclicprefix (CP) configuration. There are two types of CPs: extended CP andnormal CP. In the case of the normal CP, one slot includes 7 OFDMsymbols. In the case of the extended CP, the length of one OFDM symbolis increased and thus the number of OFDM symbols in a slot is smallerthan in the case of the normal CP. Thus when the extended CP is used,for example, 6 OFDM symbols may be included in one slot. If channelstate gets poor, for example, during fast movement of a UE, the extendedCP may be used to further decrease inter-symbol interference (ISI).

In the case of the normal CP, one subframe includes 14 OFDM symbolsbecause one slot includes 7 OFDM symbols. The first two or three OFDMsymbols of each subframe may be allocated to a physical downlink controlchannel (PDCCH) and the other OFDM symbols may be allocated to aphysical downlink shared channel (PDSCH).

FIG. 1B illustrates the type-2 radio frame structure. A type-2 radioframe includes two half frames, each having 5 subframes, a downlinkpilot time slot (DwPTS), a guard period (GP), and an uplink pilot timeslot (UpPTS). Each subframe is divided into two slots. The DwPTS is usedfor initial cell search, synchronization, or channel estimation at a UE.The UpPTS is used for channel estimation and acquisition of uplinktransmission synchronization to a UE at an eNB. The GP is a periodbetween an uplink and a downlink, which eliminates uplink interferencecaused by multipath delay of a downlink signal. One subframe includestwo slots irrespective of the type of a radio frame.

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot. A downlink slot includes 7 OFDM symbolsin the time domain and an RB includes 12 subcarriers in the frequencydomain, which does not limit the scope and spirit of the presentinvention. For example, a downlink slot may include 7 OFDM symbols inthe case of the normal CP, whereas a downlink slot may include 6 OFDMsymbols in the case of the extended CP. Each element of the resourcegrid is referred to as a resource element (RE). An RB includes 12×7 REs.The number of RBs in a downlink slot, NDL depends on a downlinktransmission bandwidth. An uplink slot may have the same structure as adownlink slot.

FIG. 3 illustrates the structure of a downlink subframe. Up to threeOFDM symbols at the start of the first slot in a downlink subframe areused for a control region to which control channels are allocated andthe other OFDM symbols of the downlink subframe are used for a dataregion to which a PDSCH is allocated. Downlink control channels used inthe 3GPP LTE system include a physical control format indicator channel(PCFICH), a physical downlink control channel (PDCCH), and a physicalhybrid automatic repeat request (HARQ) indicator channel (PHICH). ThePCFICH is located in the first OFDM symbol of a subframe, carryinginformation about the number of OFDM symbols used for transmission ofcontrol channels in the subframe. The PHICH delivers an HARQacknowledgment/negative acknowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI transports uplink ordownlink scheduling information, or uplink transmission power controlcommands for UE groups. The PDCCH delivers information about resourceallocation and a transport format for a downlink shared channel(DL-SCH), resource allocation information about an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a Random Access Responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, voice over Internet protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregatingone or more consecutive control channel elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE includes a plurality of REgroups. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a cyclic redundancycheck (CRC) to control information. The CRC is masked by an identifier(ID) known as a radio network temporary identifier (RNTI) according tothe owner or usage of the PDCCH. If the PDCCH is directed to a specificUE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE. If thePDCCH is for a paging message, the CRC of the PDCCH may be masked by apaging indicator Identifier (P-RNTI). If the PDCCH carries systeminformation, particularly, a system information block (SIB), its CRC maybe masked by a system information ID and a system information RNTI(SI-RNTI). To indicate that the PDCCH carries a random access responsein response to a random access preamble transmitted by a UE, its CRC maybe masked by a random access-RNTI (RA-RNTI).

FIG. 4 illustrates the structure of an uplink subframe. An uplinksubframe may be divided into a control region and a data region in thefrequency domain. A physical uplink control channel (PUCCH) carryinguplink control information is allocated to the control region and aphysical uplink shared channel (PUSCH) carrying user data is allocatedto the data region. To maintain the property of a single carrier, a UEdoes not transmit a PUSCH and a PUCCH simultaneously. A PUCCH for a UEis allocated to an RB pair in a subframe. The RBs of the RB pair occupydifferent subcarriers in two slots. Thus it is said that the RB pairallocated to the PUCCH is frequency-hopped over a slot boundary.

Reference Signal (RS)

In a wireless communication system, a Packet is transmitted on a radiochannel. In view of the nature of the radio channel, the Packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the received signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs may be divided into downlink RSs and uplink RSs. In the current LTEsystem, the uplink RSs include:

i) Demodulation-reference signal (DM-RS) used for channel estimation forcoherent demodulation of information delivered on a PUSCH and a PUCCH;and

ii) Sounding reference signal (SRS) used for an eNB or a network tomeasure the quality of an uplink channel in a different frequency.

The downlink RSs are categorized into:

i) Cell-specific reference signal (CRS) shared among all UEs of a cell;

ii) UE-specific RS dedicated to a specific UE;

iii) DM-RS used for coherent demodulation of a PDSCH, when the PDSCH istransmitted;

iv) Channel state information-reference signal (CSI-RS) carrying CSI,when downlink DM-RSs are transmitted;

v) Multimedia broadcast single frequency network (MBSFN) RS used forcoherent demodulation of a signal transmitted in MBSFN mode; and

vi) Positioning RS used to estimate geographical position informationabout a UE.

RSs may also be divided into two types according to their purposes: RSfor channel information acquisition and RS for data demodulation. Sinceits purpose lies in that a UE acquires downlink channel information, theformer should be transmitted in a broad band and received even by a UEthat does not receive downlink data in a specific subframe. This RS isalso used in a situation like handover. The latter is an RS that an eNBtransmits along with downlink data in specific resources. A UE candemodulate the data by measuring a channel using the RS. This RS shouldbe transmitted in a data transmission area.

Modeling of MIMO System

FIGS. 5A and 5B are diagram illustrating a configuration of a wirelesscommunication system having multiple antennas.

As shown in FIG. 5A, if the number of transmit antennas is increased toNT and the number of receive antennas is increased to NR, a theoreticalchannel transmission capacity is increased in proportion to the numberof antennas, unlike the case where a plurality of antennas is used inonly a transmitter or a receiver. Accordingly, it is possible to improvea transfer rate and to remarkably improve frequency efficiency. As thechannel transmission capacity is increased, the transfer rate may betheoretically increased by a product of a maximum transfer rate Ro uponutilization of a single antenna and a rate increase ratio Ri.

[Equation 1]

R _(i)=min(N _(T) ,N _(R))

For instance, in an MIMO communication system, which uses 4 transmitantennas and 4 receive antennas, a transmission rate 4 times higher thanthat of a single antenna system can be obtained. Since this theoreticalcapacity increase of the MIMO system has been proved in the middle of90's, many ongoing efforts are made to various techniques tosubstantially improve a data transmission rate. In addition, thesetechniques are already adopted in part as standards for various wirelesscommunications such as 3G mobile communication, next generation wirelessLAN and the like.

The trends for the MIMO relevant studies are explained as follows. Firstof all, many ongoing efforts are made in various aspects to develop andresearch information theory study relevant to MIMO communicationcapacity calculations and the like in various channel configurations andmultiple access environments, radio channel measurement and modelderivation study for MIMO systems, spatiotemporal signal processingtechnique study for transmission reliability enhancement andtransmission rate improvement and the like.

In order to explain a communicating method in an MIMO system in detail,mathematical modeling can be represented as follows. It is assumed thatthere are NT transmit antennas and NR receive antennas.

Regarding a transmitted signal, if there are NT transmit antennas, themaximum number of pieces of information that can be transmitted is NT.Hence, the transmission information can be represented as shown inEquation 2.

[Equation 2]

S=└S ₁ ,S ₂ , . . . ,S _(N) _(T) ┘^(T)

Meanwhile, transmit powers can be set different from each other forindividual pieces of transmission information S₁,S₂, . . . ,S_(N) _(T) ,respectively. If the transmit powers are set to P₁, P₂, . . . , P_(N)_(T) , respectively, the transmission information with adjusted transmitpowers can be represented as Equation 3.

[Equation 3]

Ŝ=[Ŝ ₁ ,Ŝ ₂ , . . . ,Ŝ _(N) _(T) ]^(T)=[P ₁ S ₁ , P ₂ S ₂ , . . . ,P_(N) _(T) S _(N) _(T) ]^(T)

In addition, Ŝ can be represented as Equation 4 using diagonal matrix Pof the transmission power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & & & 0 \\ & P_{2} & & \\ & & \ddots & \\0 & & & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\ \vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}4} \right\rbrack\end{matrix}$

Assuming a case of configuring NT transmitted signals X₁, X₂, . . . ,X_(N) _(T) , which are actually transmitted, by applying weight matrix Wto the information vector Ŝ having the adjusted transmit powers, theweight matrix W serves to appropriately distribute the transmissioninformation to each antenna according to a transport channel stateX₁,X₂, . . . , X_(N) _(T) can be expressed by using the vector X asfollows.

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\ \vdots \\x_{i} \\ \vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \cdots & w_{1N_{T}} \\w_{21} & w_{22} & \cdots & w_{2N_{T}} \\ \vdots & & \ddots & \\w_{i1} & w_{i2} & \cdots & w_{{iN}_{T}} \\ \vdots & & \ddots & \\w_{N_{T}1} & w_{N_{T}2} & \cdots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\ \vdots \\{\hat{s}}_{j} \\ \vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}5} \right\rbrack\end{matrix}$

In Equation 5, W_(ij) denotes a weight between an i^(th) transmitantenna and j^(th) information. W is also called a precoding matrix.

If the NR receive antennas are present, respective received signals y₁,y₂, . . . , y_(N) _(R) of the antennas can be expressed as follows.

[Equation 6]

y=[y ₁ , y ₂ , . . . , y _(N) _(R) ]^(T)

If channels are modeled in the MIMO wireless communication system, thechannels may be distinguished according to transmit/receive antennaindexes. A channel from the transmit antenna j to the receive antenna iis denoted by h_(ij). In h_(ij), it is noted that the indexes of thereceive antennas precede the indexes of the transmit antennas in view ofthe order of indexes.

FIG. 5B is a diagram illustrating channels from the NT transmit antennasto the receive antenna i. The channels may be combined and expressed inthe form of a vector and a matrix. In FIG. 5B, the channels from the NTtransmit antennas to the receive antenna i can be expressed as follows.

[Equation 7]

h_(i) ^(T)=[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]

Accordingly, all channels from the NT transmit antennas to the NRreceive antennas can be expressed as follows.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\ \vdots \\h_{i}^{T} \\ \vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & {h_{12}} & \cdots & h_{1N_{T}} \\h_{21} & h_{22} & \cdots & h_{2N_{T}} \\ \vdots & & \ddots & \\h_{i1} & h_{i2} & \cdots & h_{{iN}_{T}} \\ \vdots & & \ddots & \\h_{N_{R}1} & h_{N_{R}2} & \cdots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}8} \right\rbrack\end{matrix}$

An AWGN (Additive White Gaussian Noise) is added to the actual channelsafter a channel matrix H. The AWGN n₁, n₂, . . . , n_(N) _(R)respectively added to the NR receive antennas can be expressed asfollows.

[Equation 9]

n=[n ₁ , n ₂ , . . . ,n _(N) _(R) ]^(T)

Through the above-described mathematical modeling, the received signalscan be expressed as follows.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\ \vdots \\y_{i} \\ \vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \cdots & h_{1N_{T}} \\h_{21} & h_{22} & \cdots & h_{2N_{T}} \\ \vdots & & \ddots & \\h_{i1} & h_{i2} & \cdots & h_{{iN}_{T}} \\ \vdots & & \ddots & \\h_{N_{R}1} & h_{N_{R}2} & \cdots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\ \vdots \\x_{j} \\ \vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\ \vdots \\n_{i} \\ \vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}10} \right\rbrack\end{matrix}$

Meanwhile, the number of rows and columns of the channel matrix Hindicating the channel state is determined by the number of transmit andreceive antennas. The number of rows of the channel matrix H is equal tothe number NR of receive antennas and the number of columns thereof isequal to the number NR of transmit antennas. That is, the channel matrixH is an NR×NT matrix.

The rank of the matrix is defined by the smaller of the number of rowsand the number of columns, which are independent from each other.Accordingly, the rank of the matrix is not greater than the number ofrows or columns. The rank rank(H) of the channel matrix H is restrictedas follows.

[Equation 11]

rank(H)≤min(N _(T) , N _(R))

Additionally, the rank of a matrix can also be defined as the number ofnon-zero Eigen values when the matrix is Eigen-value-decomposed.Similarly, the rank of a matrix can be defined as the number of non-zerosingular values when the matrix is singular-value-decomposed.Accordingly, the physical meaning of the rank of a channel matrix can bethe maximum number of channels through which different pieces ofinformation can be transmitted.

In the description of the present document, ‘rank’ for MIMO transmissionindicates the number of paths capable of sending signals independentlyon specific time and frequency resources and ‘number of layers’indicates the number of signal streams transmitted through therespective paths. Generally, since a transmitting end transmits thenumber of layers corresponding to the rank number, one rank has the samemeaning of the layer number unless mentioned specially.

Synchronization Acquisition of D2D UE

Now, a description will be given of synchronization acquisition betweenUEs in D2D communication based on the foregoing description in thecontext of the legacy LTE/LTE-A system. In an OFDM system, iftime/frequency synchronization is not acquired, the resulting inter-cellinterference (ICI) may make it impossible to multiplex different UEs inan OFDM signal. If each individual D2D UE acquires synchronization bytransmitting and receiving a synchronization signal directly, this isinefficient. In a distributed node system such as a D2D communicationsystem, therefore, a specific node may transmit a representativesynchronization signal and the other UEs may acquire synchronizationusing the representative synchronization signal. In other words, somenodes (which may be an eNB, a UE, and a synchronization reference node(SRN, also referred to as a synchronization source)) may transmit a D2Dsynchronization signal (D2DSS) and the remaining UEs may transmit andreceive signals in synchronization with the D2DSS.

D2DSSs may include a primary D2DSS (PD2DSS) or a primary sidelinksynchronization signal (PSSS) and a secondary D2DSS (SD2DSS) or asecondary sidelink synchronization signal (SSSS). The PD2DSS may beconfigured to have a similar/modified/repeated structure of a Zadoff-chusequence of a predetermined length or a primary synchronization signal(PSS). Unlike a DL PSS, the PD2DSS may use a different Zadoff-chu rootindex (e.g., 26, 37). And, the SD2DSS may be configured to have asimilar/modified/repeated structure of an M-sequence or a secondarysynchronization signal (SSS). If UEs synchronize their timing with aneNB, the eNB serves as an SRN and the D2DSS is a PSS/SSS. Unlike PSS/SSSof DL, the PD2DSS/SD2DSS follows UL subcarrier mapping scheme. FIG. 6shows a subframe in which a D2D synchronization signal is transmitted. Aphysical D2D synchronization channel (PD2DSCH) may be a (broadcast)channel carrying basic (system) information that a UE should firstobtain before D2D signal transmission and reception (e.g., D2DSS-relatedinformation, a duplex mode (DM), a TDD UL/DL configuration, a resourcepool-related information, the type of an application related to theD2DSS, etc.). The PD2DSCH may be transmitted in the same subframe as theD2DSS or in a subframe subsequent to the frame carrying the D2DSS. ADMRS can be used to demodulate the PD2DSCH.

The SRN may be a node that transmits a D2DSS and a PD2DSCH. The D2DSSmay be a specific sequence and the PD2DSCH may be a sequencerepresenting specific information or a codeword produced bypredetermined channel coding. The SRN may be an eNB or a specific D2DUE. In the case of partial network coverage or out of network coverage,the SRN may be a UE.

In a situation illustrated in FIG. 7, a D2DSS may be relayed for D2Dcommunication with an out-of-coverage UE. The D2DSS may be relayed overmultiple hops. The following description is given with the appreciationthat relay of an SS covers transmission of a D2DSS in a separate formataccording to a SS reception time as well as direct amplify-and-forward(AF)-relay of an SS transmitted by an eNB. As the D2DSS is relayed, anin-coverage UE may communicate directly with an out-of-coverage UE.

D2D Resource Pool

FIGS. 8A and 8B show an example of a UE1, a UE2 and a resource pool usedby the UE1 and the UE2 performing D2D communication. In FIG. 8A, a UEcorresponds to a terminal or such a network device as an eNBtransmitting and receiving a signal according to a D2D communicationscheme. A UE selects a resource unit corresponding to a specificresource from a resource pool corresponding to a set of resources andthe UE transmits a D2D signal using the selected resource unit. A UE2corresponding to a reception UE receives a configuration of a resourcepool in which the UE1 is able to transmit a signal and detects a signalof the UE1 in the resource pool. In this case, if the UE1 is located atthe inside of coverage of an eNB, the eNB can inform the UE1 of theresource pool. If the UE1 is located at the outside of coverage of theeNB, the resource pool can be informed by a different UE or can bedetermined by a predetermined resource. In general, a resource poolincludes a plurality of resource units. A UE selects one or moreresource units from among a plurality of the resource units and may beable to use the selected resource unit(s) for D2D signal transmission.FIG. 8B shows an example of configuring a resource unit. Referring toFIG. 8B, the entire frequency resources are divided into the NF numberof resource units and the entire time resources are divided into the NTnumber of resource units. In particular, it is able to define NF*NTnumber of resource units in total. In particular, a resource pool can berepeated with a period of NT subframes. Specifically, as shown in FIGS.8A and 8B, one resource unit may periodically and repeatedly appear. Or,an index of a physical resource unit to which a logical resource unit ismapped may change with a predetermined pattern according to time toobtain a diversity gain in time domain and/or frequency domain. In thisresource unit structure, a resource pool may correspond to a set ofresource units capable of being used by a UE intending to transmit a D2Dsignal.

A resource pool can be classified into various types. First of all, theresource pool can be classified according to contents of a D2D signaltransmitted via each resource pool. For example, the contents of the D2Dsignal can be classified into various signals and a separate resourcepool can be configured according to each of the contents. The contentsof the D2D signal may include a scheduling assignment (SA), a D2D datachannel, and a discovery channel. The SA may correspond to a signalincluding information on a resource position of a D2D data channel,information on a modulation and coding scheme (MCS) necessary formodulating and demodulating a data channel, information on a MIMOtransmission scheme, information on a timing advance (TA), and the like.The SA signal can be transmitted on an identical resource unit in amanner of being multiplexed with D2D data. In this case, an SA resourcepool may correspond to a pool of resources that an SA and D2D data aretransmitted in a manner of being multiplexed. The SA signal can also bereferred to as a D2D control channel or a physical sidelink controlchannel (PSCCH). The D2D data channel (or, physical sidelink sharedchannel (PSSCH)) corresponds to a resource pool used by a transmissionUE to transmit user data. If an SA and a D2D data are transmitted in amanner of being multiplexed in an identical resource unit, D2D datachannel except SA information can be transmitted only in a resource poolfor the D2D data channel. In other word, REs, which are used to transmitSA information in a specific resource unit of an SA resource pool, canalso be used for transmitting D2D data in a D2D data channel resourcepool. The discovery channel may correspond to a resource pool for amessage that enables a neighboring UE to discover transmission UEtransmitting information such as ID of the UE, and the like.

Although contents of D2D signal are identical to each other, it may usea different resource pool according to a transmission/receptionattribute of the D2D signal. For example, in case of the same D2D datachannel or the same discovery message, the D2D data channel or thediscovery signal can be classified into a different resource poolaccording to a transmission timing determination scheme (e.g., whether aD2D signal is transmitted at the time of receiving a synchronizationreference signal or the timing to which a prescribed timing advance isadded) of a D2D signal, a resource allocation scheme (e.g., whether atransmission resource of an individual signal is designated by an eNB oran individual transmission UE selects an individual signal transmissionresource from a pool), a signal format (e.g., number of symbols occupiedby a D2D signal in a subframe, number of subframes used for transmittinga D2D signal), signal strength from an eNB, strength of transmit powerof a D2D UE, and the like. For clarity, a method for an eNB to directlydesignate a transmission resource of a D2D transmission UE is referredto as a mode 1. If a transmission resource region is configured inadvance or an eNB designates the transmission resource region and a UEdirectly selects a transmission resource from the transmission resourceregion, it is referred to as a mode 2. In case of performing D2Ddiscovery, if an eNB directly indicates a resource, it is referred to asa type 2. If a UE directly selects a transmission resource from apredetermined resource region or a resource region indicated by the eNB,it is referred to as type 1.

SA Transmission/Reception

A mode-1 UE may transmit an SA (or D2D control signal) and sidelinkcontrol information (SCI) in resources configured by an eNB. For amode-2 UE, the eNB configures resources for D2D communication. Themode-2 UE may select time-frequency resources from the configuredresources and transmit an SA in the selected time-frequency resources.

A SA period may be defined as illustrated in FIG. 9. Referring to FIG.9, a first SA period may start in a subframe spaced from a specificsystem frame by a predetermined offset, SAOffsetlndicator indicated byhigher-layer signaling. Each SA period may include an SA resource pooland a subframe pool for D2D data transmission. The SA resource pool mayinclude the first subframe of the SA period to the last of subframesindicated as carrying an SA in a subframe bitmap, saSubframeBitmap. Theresource pool for D2D data transmission may include subframes used foractual data transmission through application of a time-resource patternfor transmission (T-RPT) or a time-resource pattern (TRP) in mode 1. Asillustrated, if the number of subframes included in an SA period exceptfor an SA resource pool is larger than the number of T-RPT bits, theT-RPT may be applied repeatedly, and the last applied T-RPT may beapplied truncated by as much as the number of remaining subframes.

Now, a description will be given of methods for acquiring time andfrequency synchronization in D2D communication, particularly forcommunication between vehicles, between a vehicle and another terminal,and between a vehicle and an infrastructure network, based on the abovedescription. The foregoing method related to a D2DSS is characterized inthat priority is given to synchronization provided by a network. Morespecifically, a UE selects an SS transmitted directly by an eNB with ahighest priority, in determining its transmission synchronization. Ifthe UE is located outside the coverage of the eNB, the UE is firstsynchronized with a D2DSS transmitted by a UE within the coverage of theeNB. This operation is intended to bring about the effect of reliablemultiplexing between a D2D operation and a legacy network operation(transmission and reception between an eNB and a UE) throughsynchronization of a UE with a timing provided by the network, ifpossible (e.g., the legacy network operation is performed in onesubframe, and D2D communication is conducted in the next subframe).Meanwhile, a wireless terminal installed in a vehicle or a terminalmounted in a vehicle may not experience a relatively great batteryconsumption problem, and may use a satellite signal such as a globalpositioning system (GPS) signal for the purpose of navigation.Accordingly, the satellite signal is available for time or frequencysynchronization between terminals. Besides a GPS signal, the satellitesignal may be any of a global navigation satellite systems (GNSS)signal, a global navigation satellite system (GLONAS) signal, a GALILEOsignal, a BEIDOU signal, and so on. While the following description isgiven mainly in the context of a GNSS signal and a GPS signal assatellite signals, by way of example, the satellite signals may bereplaced with other satellite signals. Further, a vehicle (V)-UE may bea UE moving in a vehicle, and a pedestrian (P)-UE may be a UE moving onfoot or on a cycle in the following description. Further, a GPS timingmay refer to configuring a frame/subframe boundary based on an absolutetime being a time acquired by GPS reception (e.g., a coordinateduniversal time (UTC) time or a GPS time) and then configuring a part orall of subframes as subframes for D2D signal transmission. A cellulartiming means a frame/subframe boundary for a D2D signal, generated basedon a time obtained by applying a predetermined offset (e.g., a timingadvance) to a reception time of a PSS/SSS or SLSS transmitted by anadjacent eNB or RSU (e.g., an eNB or RSU from which a signal having alargest RSRP is received) or a reception time of a PSS/SSS transmittedby an eNB. A radio frame/subframe boundary may be set by applying apredetermined offset (the offset may be 0 under circumstances) to thePSS/SSS reception time and then some subframes may be configured as D2Dsubframes. In the following description, SLSS id_net may represent a setof SLSS IDs available for UEs which have selected an SS of an eNB as asynchronization reference, from among physical-layer SLSS IDs {0, 1, . .. , 335}. SLSS id_net may include SLSS IDs {168, 169, . . ., 335}.

Transmission/Reception and Priority of Sidelink Synchronization Signal(SLSS)

In a situation where a GNSS co-exists with an eNB, how a UE prioritizesSSs/synchronization sources in selecting an SS/synchronization sourceand transmitting an SS may be an issue. Now, a description will be givenof various embodiments regarding the prioritization. Each entitydescribed in the following description may be as illustrated in FIG. 10.Specifically, referring to FIG. 10, a GNSS, a GNSS-based UE (UE G-1), aneNB-based UE (UE N-1), a two-hop GNSS-based UE (UE G-2), a two-hopeNB-based UE (UE N-2), and an out-of-network (OON) UE are shown in FIG.10. In FIG. 10, a solid line indicates that a signal is receivabledirectly from a corresponding transmission entity, which may be over 1hop.

GNSS-based UE=eNB-based UE

An eNB-based UE may have the same priority as that of a GNSS-based UE.Regarding SS transmission in this case, an in-coverage UE may select anSLSS ID and transmit an SLSS generated based on the selected SLSS ID.The SLSS ID may be selected from the same SLSS ID set (e.g., SLSSid_net) as used for SLSS transmission of a UE which has received aPSS/SSS directly from an eNB and selected synchronization with the eNBas a timing and/or frequency reference (i.e. an eNB-based UE). Herein,for a GNSS UE, one SLSS ID may be selected from SLSS id_net (a set ofSLSS IDs selected by eNB-based UEs). This ID may be preset or signaledby the network. A UE which has received a signal directly from the GNSS,has synchronized with the GNSS based on the received signal, and hasselected the GNSS as a timing and/or frequency reference (i.e., aGNSS-based UE) may transmit an SLSS in the same resources as used forSLSS transmission of a UE which has received a PSS/SSS directly from theeNB, or in resources configured separately for GNSS-based UEs, and mayuse the same PSBCH field as a predetermined PSBCH field used for SLSStransmission by the UE which has received the PSS/SSS directly from theeNB. The predetermined PSBCH field is a coverage indicator field, andthe value of the coverage indicator field may be set to 1.

That is, as the eNB-based UE and the GNSS-based UE use SSIDs (of thesame level) (and/or the same resources and the same coverage indicators)selected from an SLSS ID set used by high-priority UEs, SLSSstransmitted by the eNB-based UE and the GNSS-based UE may be regarded asequal signals (i.e., signals with the same priority) by a receiving UE.In this case, the receiving UE may select an SLSS having a largerRSRP/sidelink RSRP (S-RSRP) as a synchronization source (a UE which hastransmitted an SLSS having a large S-RSRP is selected as asynchronization source from among UEs which has received SLSSs directlyfrom a UE and the eNB).

The above prioritization of an eNB-based UE and a GNSS-based UE into thesame priority level may prevent a UE from selecting a UE transmitting apoor SS as a synchronization source. It is assumed that a higherpriority is given to the eNB-based UE than the GNSS-based UE. Further,it is assumed that UE X receives an SS from each of UE G-1 (theGNSS-based UE) and UE N-1 (the eNB-based UE). In this case, an SLSStransmitted by the nearer UE G-1 will have a higher S-RSRP than an SLSStransmitted by UE N-1. However, UE X should select UE N-1 which is lowerin S-RSRP but higher in priority than UE G-1, according to the priorityof UE N-1. In this case, UE X may have difficulty in acquiring accuratesynchronization. Moreover, if UE X which has acquired inaccuratesynchronization transmits an SLSS based on the inaccuratesynchronization, the SLSS may act as severe interference with other UEsreceiving an SS from the GNSS. Therefore, the above prioritization ofthe eNB-based UE and the GNSS-based UE into the same priority level mayovercome this problem. If the eNB-based UE and the GNSS-based UE havethe same priority, an in-coverage UE selects a UE having the higherS-RSRP between the eNB-based UE and the GNSS-based UE as asynchronization source, and thus adjacent UEs commonly have the samesynchronization source. In this case, the problem of interference causedby an SLSS transmitted by an asynchronous UE does not occur. In theabove description, a GNSS signal may be receivable/available to the eNB.If a UE detects the eNB, the eNB may signal whether the eNB has priorityover the GNSS or vice versa. Although it is preferable to give priorityto the GNSS over the eNB for the purpose of decreasing a frequencyoffset, the eNB may have priority over the GNSS according to signaling.

GNSS based UE=eNB based UE >two hop GNSS based UE=two hop eNB basedUE >OON UE

As described above, if the eNB-based UE and the GNSS-based UE have thesame priority, it is natural that a two-hop eNB-based UE and a two-hopGNSS-based UE have the same priority. That is, the two-hop eNB-based UEand the two-hop GNSS-based UE may use the same all or part of resources,SLSS IDs, and PSBCH incoverage indicators. That is, the two-hopGNSS-based UE may use coverage indicator=0, SLSS id_net (reserved forthe GNSS), and SLSS resources reserved for two-hop GNSS-based UEs.Likewise, the two-hop GNSS-based UE may use a preset id from SLSSid_net, an id signaled by the network, or an SLSS id of a GNSS-based UEwhich the two-hop GNSS-based UE has selected as a synchronizationreference. SLSS resources used by the two-hop GNSS-based UE may beindicated by a PSBCH or the positions of the SLSS resources may bepreset.

In consideration of all of the above descriptions, prioritization may beperformed in the order of ‘eNB >GNSS>GNSS based UE=eNB based UE >two hopGNSS based UE=two hop eNB based UE >OON UE’ or ‘GNSS>eNB>GNSS basedUE=eNB based UE >two hop GNSS based UE=two hop eNB based UE >OON UE’ (inthis case, there may be no signaling for an additional priority. Or asignal indicating a GNSS signal is receivable in the eNB may be used forthe purpose of indicating the above priorities).

GNSS >(direct)GNSS-based UE

For an out-of-coverage UE, the GNSS may have a highest priority. Then,priorities may be determined in consideration of whether a GNSS signalis receivable, a required frequency offset in the case of GNSSreception, and the number of hops. In a situation where the GNSS is notdetected (in a tunnel or under a high pass), it is most reasonable tosynchronize with an SLSS transmitted by a UE which has received a GNSSsignal directly in that the GNSS timing is maintained. Therefore, a(direct) GNSS-based UE may have a priority next to the GNSS.

GNSS >(direct)GNSS-based UE >eNB-based UE >two-hop GNSS-basedUE >two-hop eNB-based UE

This prioritization may be for the case where a GNSS signal isreceivable in the eNB. This considers whether the eNB has a GNSSreception capability, how tight a frequency offset requirement is, andso on. Specifically, in the case where the eNB is capable of receiving aGNSS signal and a frequency offset is commonly 0.1 ppm when the eNBreceives a GNSS signal and when a UE receives a GNSS signal directly,the two-hop GNSS-based UE and the eNB-based UE have the same number ofhops, two hops from the GNSS and the same expected frequency offsetrequirement. Considering that the eNB is always fixed and has arelatively small frequency offset, it is configured that the eNB-basedUE has a higher priority than the two-hop GNSS-based UE.

GNSS >(direct)GNSS-based UE >eNB-based UE >Two-Hop GNSS-basedUE >Two-hop eNB-based UE >Three-Hop GNSS-based UE >OON UE

If an OON UE is defined to be two or more hops away, the aboveprioritization may be regulated.

GNSS >(direct)GNSS-based UE >eNB-based UE=Two-Hop GNSS-based UE >Two-hopeNB-based UE >OON UE

This prioritization is characterized in that the eNB-based UE has thesame priority as the two-hop GNSS-based UE. The prioritization is basedon the consideration that the eNB-based UE and the two-hop GNSS-based UEare the same number of hops away from the GNSS and do not have a largetiming difference. That is, as the same priority is assigned to theeNB-based UE and the two-hop GNSS-based UE, the eNB-based UE and thetwo-hop GNSS-based UE are allowed to use the same resources/SLSSID/PSBCH, thereby achieving a single frequency network (SFN) effect (asdifferent UEs transmit the same signal in the same resources, the signalis stably transmitted in the resources (with high reception power)) andenabling a later configuration without prioritization.

In the above prioritization, the eNB-based UE uses a network-signaled idfrom id_net and coverage indicator=1, for an SLSS. On the assumptionthat the two-hop GNSS-based UE uses the same id and PSBCH, theGNSS-based UE may use an SLSS id selected from id_net and coverageindicator=1, with only resources distinguished. However, in the case,the GNSS-based UE and the two-hop GNSS-based UE may not be distinguishedfrom each other. In another method, although the GNSS-based UE usescoverage indicator=1, the GNSS-based UE may use an id reserved for theGNSS from in oon, or a PSSS with a different sequence ID. In this case,a one-hop GNSS-based UE may be distinguished from a two-hop GNSS-basedUE by SLSS IDs. Under circumstances, the GNSS-based UE may use coverageindicator=0. That is, irrespective of coverage, the GNSS-based UE mayuse coverage indicator=0, and use a reserved id from id_oon or adifferent PSSS ID. The eNB-based UE and the two-hop GNSS-based UE mayuse coverage indicator=1, an SLSS ID from_id net, and the same SLSSresources.

Regarding the above prioritization, in the case of in-coverage, that is,in the case where a UE detects the eNB, the priorities may be in theorder of eNB >GNSS >(direct) GNSS-based UE >eNB-based UE=two-hopGNSS-based UE >two-hop eNB-based UE >OON UE, or GNSS >eNB >(direct)GNSS-based UE >eNB-based UE=two-hop GNSS-based UE >two-hop eNB-basedUE >OON UE.

If the eNB is not capable of receiving a GNSS signal, it is preferablein terms of continuity of a V2V operation to assign a lower priority tothe eNB than the GNSS. In consideration of this, the priorities may bein the order of i) GNSS>(direct) GNSS based UE, ii) GNSS>(direct) GNSSbased UE>two hop GNSS based UE >eNB based UE>two hop eNB based UE >OONUE , iii) eNB>GNSS>(direct) GNSS based UE>two hop GNSS based UE >eNBbased UE>two hop eNB based UE >OON UE, iv) eNB based UE >two hop eNBbased UE >GNSS>GNSS based UE >two hop GNSS based UE >indirect (more thanone hop) GNSS based UE>OON UE, v) GNSS>(direct) GNSS based UE>two hopGNSS based UE >eNB>eNB based UE>two hop eNB based UE >OON UE. Herein,some priorities may not be used. For example, if the eNB assigns ahighest priority to the GNSS, an eNB-based synchronization source (eNB,a one-hop eNB-based UE, and a two-hop eNB-based UE) may be excluded fromprioritization. As a result, timing discontinuity between heterogeneoussynchronization sources may be eliminated, while only GNSS-basedsynchronization sources are used. Herein, exclusion from prioritizationmay mean that corresponding synchronizations signals are ignored eventhough the synchronization signals are received, or the eNB commands theeNB-based UE or the two-hop eNB-based UE not to transmit a D2DSS.

In the above description, the eNB should be able to fully control SLSStransmissions of in-coverage UEs. GNSS UEs may use reserved id_net(e.g., 0 or 167). If the eNB is capable of acquiring synchronizationwith the GNSS, the eNB may use id_net=167 and coverage indicator=1 for aUE (as if the GNSS were one cell). Regarding SLSSs transmitted by UEs,an SLSS with a higher priority is used. If the SLSSs have the samepriority, an SLSS with a higher S-RSRP is used. Or if the eNB is capableof acquiring synchronization with the GNSS, the eNB may command aneNB-based UE to use the same SLSS ID as an id reserved for GNSS UEs andthe same synchronization resources. This case may be intended to achievethe SFN effect by enabling the eNB-based UE and the GNSS-based UE to usethe same SLSS.

[Table 1] below illustrates exemplary priorities and PSBCH and SLSS Idsettings in relation to GNSS reception.

TABLE 1 Case 1 Priority GNSS > eNB based UE = GNSS based UE > two hopeNB based UE = two hop GNSS based UE > OON UE PSBCH i. For GNSS-basedUE, coverage indicator = and 1 and SLSS ID from in_net (id_net may useSLSS Id ids configured for the GNSS commonly setting by all cells). ii.For two-hop GNSS-based UE, coverage indicator = 0 and SLSS ID fromin_net (id_net may use ids configured for the GNSS commonly by allcells). iii. For others, id_oon and coverage indicator = 0. Case 2Priority GNSS > GNSS based UE > eNB based UE = two hop GNSS based UE >two hop eNB based UE > OON UE PSBCH i. For two-hop GNSS-based UE,coverage and indicator = 1 and SLSS ID from id_netii. SLSS Id ForGNSS-based UE, coverage indicator = 0 setting and SLSS ID from id_net.Or a little uniquely, for GNSS-based UE, coverage indicator = 1 and SLSSID from id_oon, or a newly produced indicator. ii. For GNSS- based UE,coverage indicator = 0 or 1 and SLSS ID from id_net. To indicate GNSSdirect, a new indicator is included using a reserved bit of a new PSBCH.Case 3 Priority GNSS > GNSS based UE > eNB based UE > two hop GNSS basedUE > two hop eNB based UE > OON UE PSBCH For eNB-based UE and two-hopeNB-based and UE, the same as conventional (for eNB-based SLSS Id UE,coverage indicator = 1, and for setting two-hop eNB-based UE, coverageindicator = 0). For GNSS-based UE, coverage indicator = 1 and SLSS IDfrom id_net, and for two- hop GNSS-based UE, coverage indicator = 0 andSLSS ID from id_net, (id_net may use ids configured for the GNSScommonly by all cells). Case 4 Priority GNSS > GNSS based UE > two hopGNSS based UE > eNB based UE > two hop eNB based UE > OON UE (for thisoperation, a high priority should be assigned to an SLSS ID used by theGNSS. That is, it may be regulated that if a specific SLSS ID isdetected, the SLSS ID has a higher priority, irrespective of whether thecoverage indicator is 0 or 1. Among SLSS IDs (used by the GNSS), an SLSSID with coverage indicator = 1 has a higher priority because the SLSS IDis for a direct GNSS (GNSS- based) UE. Case 5 Priority GNSS > eNB basedUE > two hop eNB based UE > GNSS based UE > two hop GNSS based UE > OONUE (in this case, it is assumed that there is a nearby eNB. However, theGNSS has a highest priority not to breach the agreement of 3GPP RAN182bis that the GNSS has a highest priority out of coverage). Case 6Priority eNB based UE > two hop eNB based UE > GNSS > GNSS based UE >two hop GNSS based UE > OON UE (for this prioritization, it is necessaryto signal which type of synchronization has priority to anout-of-coverage UE. It may be signaled whether the GNSS type (GNSS,GNSS-based UE, and two-hop GNSS-based UE) or the eNB type (eNB,eNB-based UE, and two-hop eNB-based UE) has priority by means of a PSBCHreserved bit. Further, this indication bit may be preconfigured to aspecific state). Case 7 Priority The priorities of GNSS-based andeNB-based are determined using the S-RSRPs of received signals (in thiscase, all SLSS IDs are regarded as having the same priority, withoutprioritization, and it is determined whether GNSS-based or eNB-based haspriority, using S-RSRP).

The PSBCH and SLSS Id settings of Case 4 to Case 7 may be similar tothat of Case 3. Case 1 to Case 3 may correspond to the case where theeNB is capable of GNSS reception, and Case 4 to Case 7 may correspond tothe case where the eNB is not capable of GNSS reception. That is,priorities may be defined depending on whether the eNB is capable ofGNSS reception. In the case of out-of-coverage, it may be preferablethat GNSS >GNSS-based UE.

[Table 2] below illustrates an exemplary relationship between PSBCHsettings (coverage indicator settings) and SLSS ID settings.

TABLE 2 Case 1 SLSS ID GNSS-based UE and two-hop GNSS-based UE use asetting predetermined SLSS (a specific ID from SLSS id_net may bereserved for GNSS usage. Coverage Coverage indicator = 1 for GNSS-basedUE, and indicator coverage indicator = 0 for two-hop GNSS-based UE.setting Others The same as conventional for eNB-based UE. Coverageindicator = 1 for eNB-based UE, and coverage indicator = 0 for two-hopeNB-based UE. Case 2 SLSS ID GNSS-based UE: SLSS ID reserved for thesetting GNSS from id_oon. Two-hop GNSS-based UE: id_net having the sameSSSS as the foregoing id_oon among id_net. Coverage GNSS-based UE:coverage indicator = 0. Two-hop indicator GNSS-based UE: coverageindicator = 1. setting Others Prioritization rule: a higher priority maybe assigned to a signal from Id_oon with coverage indicator = 1 thanother UE types. Case 3 SLSS ID GNSS-based UE and two-hop GNSS-based UE:setting SLSS ID reserved for the GNSS from id_net. Coverage GNSS-basedUE and two-hop GNSS-based UE: indicator coverage indicator = 1. settingOthers PSBCH reserved bit: GNSS direct or GNSS indirect is indicated bythe reserved bit, or a field indicating the count of hops from the GNSSis indicated by the reserved bit. If the eNB is capable of GNSSreception, the PSBCH reserved bit may be appropriately set, for SFN withPSBCHs transmitted by GNSS UEs. If the PSBCH bit is set to the same forthe eNB-based UE and the GNSS-based UE, the eNB-based UE and theGNSS-based UE have the same priority. If the PSBCH bit is set to thesame for the two-hop eNB- based UE and the GNSS-based UE, the two-hopeNB-based UE and the GNSS-based UE have the same priority. In thismanner, the eNB may establish a priority relationship with GNSStiming-based SLSSs by appropriately setting SLSS IDs, the coverageindicator, and the PSBCH reserved bit.

[Table 3] below illustrates exemplary priorities in terms ofprioritization.

TABLE 3 Case 1 GNSS > GNSS based UE > two hop GNSS based UE > eNB basedUE > two hop eNB based UE > oon UE SLSS used by the GNSS is prioritized.Case 2 GNSS > GNSS based UE = eNB based UE > two hop GNSS based UE = twohop eNB based UE > oon UE Case 3 GNSS > GNSS based UE > two hop GNSSbased UE = eNB based UE > two hop eNB based UE > oon UE All or a part ofthe coverage indicator, PSBCH reserved bit, and id setting methods maybe set differently between the GNSS-based UE and the two-hop GNSS-basedUE to distinguish them from each other. Case 4 GNSS > eNB based UE > twohop eNB based UE > GNSS based UE > two hop eNB based UE > oon UE SLSSused by the GNSS is deprioritized. A prioritization relationship may besignaled by the network, on a PSBCH by an in-coverage UE, orpreconfigured. Case 5 GNSS > eNB based UE > GNSS based UE > two hop eNBbased UE > two hop based GNSS based UE > oon UE Prioritized according toSLSS ids in spite of the same coverage indicator. Case 6 GNSS > GNSSbased UE > eNB based UE > two hop based GNSS based UE > two hop eNBbased UE > oon UE Prioritized according to SLSS ids in spite of the samecoverage indicator. Case 7 eNB based UE > two hop eNB based UE > GNSS >GNSS based UE > two hop GNSS based UE > oon UE Case 8 eNB based UE >GNSS > GNSS based UE >, = two hop eNB based UE >, = two hop GNSS basedUE > oon UE Case 9 eNB based UE > GNSS > two hop eNB based UE >, = GNSSbased UE >, = two hop GNSS based UE > oon UE

In the above table, >,=means an equal or higher priority, and an equalpriority means that prioritization is performed based on S-RSRPs. In theabove table, a specific synchronization source may be excluded frompriority levels. Priority may be determined between the eNB-based UE andthe GNSS-based UE or between the eNB and the GNSS by comparing an RSRPand an S-RSRP from the eNB or an RSRP from the eNB and the receptionquality of a GNSS signal (or a value obtained by applying a specificoffset to each measurement metric. Herein, the offset applied to eachmetric may be preset or signaled to a UE in a physical-layer signal or ahigher-layer signal by the network). This operation is advantageous inthat a synchronization problem in an asynchronous network may be solvedwith the assistance of the GNSS and no additional priority signaling isneeded by following an eNB timing in coverage where a signal from theeNB is strong and following a GNSS timing between cells.

Regarding priority between the eNB and the GNSS, if the eNB considers acellular operation as important, it may be indicated that the eNB haspriority over the GNSS. If the GNSS-based UE has a high priority, asynchronization reference may be changed too often. Therefore, the eNBmay have a higher priority than the GNSS-based UE. In this case, thenetwork may indicate the priority of the eNB. If out-of-coverage UEsregard the GNSS as having a highest priority, cell-edge cellular UEs maybe interfered by an out-of-coverage D2D operation. Accordingly, in thiscase, the eNB-based UE may have a higher priority than the GNSS as inCase 8. If the same priority is assigned to the eNB-based UE and theGNSS-based UE, the eNB-based UE=the GNSS-based UE eventually has ahigher priority than the GNSS. Herein, among GNSS-based UEs, differentSLSS ids may be configured between an out-of-coverage UE receiving aGNSS signal and an in-coverage UE receiving a GNSS signal. For example,the in-coverage UE receiving a GNSS signal uses a preset ID from SLSSid_net, and the out-of-coverage UE receiving a GNSS signal uses a presetID from SLSS id_oon. Then, even though the out-of-coverage UE is aGNSS-based UE, the UE may have a lower priority than the GNSS. That is,priorities may be set in the order of eNB based UE=GNSS based UE with incoverage>GNSS >GNSS based UE in out coverage >two hop GNSS based UE >OOCUE synched to two hop UEs >other (standalone) OOC UEs. This is aprioritization for cancelling interference of a cell-edge UE through acombination of the above cases.

Multi-Carrier Situation (Operation in Different Carriers)

In the foregoing description, although the eNB may be detected in acarrier in which a V2V operation is performed, the eNB may be detectedin any other carrier (e.g., a legacy LTE carrier). In the presence ofmultiple carriers, priority may be determined/set between the GNSS andthe eNB on a carrier basis. If the priorities of the GNSS and the eNBare not set on a carrier basis, the out-of-coverage prioritization(i.e., the highest priority is assigned to the GNSS) should be followedin a cell in which the eNB is not deployed. In this case, the eNB timingis used for a carrier in which the eNB is deployed, and the GNSS timingis used for a carrier in which the eNB is not deployed (in a carrieraggregation situation), thereby causing a timing difference between thetwo carriers. Since a UE sets power on a subframe basis in this case,the UE should determine transmission power in advance in considerationof power available for a subsequent subframe in the case where asubframe is partially overlapped between the two carriers. Therefore,the UE may not fully use transmission power in each subframe. In otherwords, in order to reduce a transmission power mismatch in a subframecaused by the timing difference, the transmission power should bereduced. Therefore, this problem may be overcome by assigning prioritieson a carrier basis. For example, even though an operator performs a D2Doperation for a specific UE in a carrier in which the eNB is notdeployed, the UE may be configured to acquire subframe synchronizationusing an eNB signal of another carrier.

The eNB may signal priorities determined for each carrier. If the timingof the eNB detected in a carrier in which a V2V operation does not takeplace is used, an eNB-based synchronization priority may be set to belower than a GNSS-based synchronization priority. If the eNB is deployedin both a carrier in which a V2V operation takes place and an LTEcarrier, the eNB may have a higher priority in the carrier in which aV2V operation takes place than in the LTE carrier.

If there is additional priority signaling regarding per-carriersynchronizations, corresponding priorities may be followed. That is, theLTE carrier may have a higher priority than the V2V carrier in somecases, which is done to protect operations in the legacy LTE carrier.

More specifically, the afore-described priorities may be determinedseparately for carrier A in the order of X>Y>Z and for carrier B in theorder of Z>Y>Z? (herein, X, Y and Z represent synchronization sources inthe afore-described synchronization prioritizations). To determine thesepriorities, the network may indicate per-carrier synchronizationpriorities to a UE by a physical-layer signal or a higher-layer signal.

If the UE fails to receive the signaling, the UE may follow theout-of-coverage prioritization, which may be preset.

Different Operators Situation (Different Operators Operation)

In the case where two UEs, UE A and UE B belong to different operatorsand the operators have different eNB timings, even though the UEs areeNB-based UEs, the UEs have different timings. A specific operator maydeploy a GNSS-enabled eNB, and the other operator may deploy aGNSS-disabled eNB. In this case, GNSS timing information may betransmitted between to the two operators through signaling betweennetworks, or a UE feeds back information indicating detection of aGNSS-based SLSS to its operator so that the GNSS-disabled eNB may re-seta subframe boundary, thereby reducing the effects of a V2V operation.

Meanwhile, even though the eNB of a specific carrier is capable ofreceiving a GNSS signal, if the eNB of another specific carrier fails toreceive a GNSS signal, a D2D subframe may not be used at a UTC timingbased on the GNSS. Generally, if the eNB fails to use the D2D subframeconfigured at the GNSS-based UTC timing, the eNB may signal an offsetbetween the timing used by the eNB and the UTC timing to a UE by aphysical-layer signal or a higher-layer signal.

Meanwhile, while an eNB implicitly signals the use of a GNSS in thepropose methods, the eNB may explicitly signal the use of a GNSS to a UEby a physical-layer signal or a higher-layer signal. Thus, the freedomof setting an SLSS at the eNB may be advantageously increased.

Another embodiment regarding synchronization prioritization will bedescribed below with reference to FIG. 10.

Synchronization prioritization may be performed in consideration of thefollowings.

i) Hop count from GNSS (a smaller hop count has a higher priority, and amaximum hop count is limited as in LTE Release 12). ii) Frequency offset(a frequency offset is related to hop counts from the eNB, a UE, and theGNSS). iii) Priority indication from the eNB (the network may explicitlyor implicitly indicate which one between the eNB timing and the GNSStiming has a higher priority). iv) GNSS reception capability of the eNB(if the eNB does not have the GNSS reception capability, the GNSS timingmay have a higher priority than the eNB timing, for a better V2Voperation). vi) Influence on Uu (related to the GNSS receptioncapability of the eNB. Cell-edge UE performance and a Uu operationshould be considered).

Synchronization priorities for out-of-coverage UEs are first considered.The GNSS has a highest priority. Then, in the in-coverage case, thepriorities of the GNSS-based UE (UE G-1) and the eNB-based UE (UE N-1)need to be determined.

If the eNB has the GNSS reception capability, UE G-1 is one hop awayfrom the GNSS, and UE N-1 is two hops away from the GNSS. Accordingly,UE G-1 may have a higher priority than UE N-1. However, if the eNB hasthe GNSS reception capability, the eNB may have a very small frequencyoffset requirement, and UE G-1 may have the same priority as UE N-1. Thesame priority means that if a plurality of synchronization sources havethe same priority, a synchronization source is selected based on S-RSRPmeasurements.

Another issue is to determine whether priority determined between UE G-1and UE N-1 interferes with Uu. If UE G-1 has a higher priority than UEN-1, the cell-edge performance of a TDD cell may be degradedsignificantly, and V2V operations may not be subjected to TDM reliablyin FDD. This issue is also related to priority between the GNSS and UEN-1. However, the GNSS has the highest priority outside coverage. Toassign a higher priority UE-N1 than the GNSS, an agreement should bechanged. In summary, the priorities may be determined in the order ofGNSS >UE G-1>UE N-1, GNSS >UE G-1=UE N-1, or UE N-1>GNSS >UE G-1 (if theagreement is changed so that the eNB can prioritize synchronizationsources originated from the eNB, and the network has (pre)configuredthat an eNB-based synchronization reference is a higher priority thanthe GNSS). Subsequently, priority needs to be determined between thetwo-hop GNSS-based UE (UE G-2) and the eNB-based UE (UE N-1).

When the eNB has the GNSS reception capability, UE G-2 is two hops awayfrom the GNSS, and UE N-1 is also two hops away from the GNSS. Thisimplies that UE G-2 and UE N-1 have the same priority. On the otherhand, the eNB is always at a fixed position, and UE G-1 is movable.Thus, since the total frequency offset of UE N-1 is smaller than that ofUE-G2, UE-N1 has a higher priority than UE-G2. If the eNB does not havethe GNSS reception capability, UE N-1 and UE G-2 may be prioritizedbased on a network configuration. If the network always puts GNSS-basedsynchronization first, UE G-2 may have a higher priority than UE N-1.Accordingly, UE N-1 and UE G-2 may be prioritized as follows: UE N-1 =UEG-2, UE N-1 >UE G-2, or UE G-2 >UE N-1.

A similar analysis may be applied to determination of priority betweenUE N-2 and UE G-2. It is natural that the OON UE has a lowest priority.

A case in which an in-coverage UE may detect a signal from the eNB isconsidered. The network may prioritize eNB-based synchronization andGNSS-based synchronization. Even though the network assigns a higherpriority to the eNB-based synchronization than the GNSS-basedsynchronization, if the UE is out of coverage, the prioritization of theeNB may be nullified due to the legacy agreement (RANI #82bis). Hence,unless the agreement is amended, the eNB-based synchronization prioritymay not be achieved perfectly. Moreover, the eNB may appropriately setan SLSS ID and PSBCH content, and there may be no need for explicitsignaling for the GNSS-based synchronization priority, which will bedescribed later.

In summary, one of various prioritizations such as options listed in[Table 4] may be used for out-of-coverage UEs.

TABLE 4 Option Priority order option 1 GNSS > UE G-1 > UE N-1 > UE G-2 >UE N-2 > OON UE option 2 GNSS > UE G-1 = UE N-1 > UE G-2 = UE N-2 > OONUE option 3 GNSS > UE G-1 > UE N-1 = UE G-2 > UE N-2 > OON UE option 4UE N-1 > UE N-2 > GNSS > UE G-1 > UE G-2 > OON UENote: modifyingagreement is necessary. option 5 GNSS > UE G-1 > UE G-2 > UE N-1 > UEN-2 > OON UENote: GNSS synchronization prioritization is preconfiguredor signaled via PSBCH.

SLSS ID and PSBCH settings may be given as follows.

It may be preferred to separate a GNSS-based SLSS ID from an eNB-basedSLSS ID. To minimize the influence on the legacy technology, it ispossible to reserve an existing SLSS ID as a GNSS-based SLSS ID. Herein,the eNB does not prohibit an in-coverage UE from transmitting aGNSS-based SLSS ID. For example, if the eNB is capable of receiving aGNSS signal, the eNB may command the in-coverage UE to use a GNSS-basedSLSS ID.

A PSBCH field may be set differently according to a priority option. InOption 2, UE G-1 and UE N-1 have the same priority. In this case, ifcoverage indicator=1 for UE G-1, UE N-1 and UE G-1 may be SFNed. Ifdifferent types of SSs are SFNed, each UE does not need to performseparate measurements for synchronization measurements, therebysimplifying UE implementation. [Table 5] below lists SLSS ID and PSBCHsettings in each option of [Table 4].

Option 1 and Option 2 do not request use of the PSBCH reserved bit. Thedifference between Option 1 and Option 2 is whether the GNSS isprioritized in the same hop count. In Option 2, a UE determines asynchronization source based on an S-RSRP measurement. On the otherhand, the GNSS is prioritized in the same hop count in Option 1. Todescribe more specific issues of Option 1 and Option 2, a situation inwhich a UE can see two UEs, UE N-1 and UE G-1 within coverage, and isnearer to UE N-1 is considered. In Option 1, a receiving UE prioritizesUE N-1. Any option should be able to deal with a multiple timing issuewhen the eNB is not capable of receiving a GNSS signal.

TABLE 5 Option SLSS ID and PSBCH setting for UE G-1 and UE G-2 optionPriority: GNSS > UE G-1 > UE N-1 > UE G-2 > UE N-2 > 1 OON UESLSS ID forUE G-1: An ID in in_net is reserved for GNSS basedsynchronizationCoverage indicator for UE G-1: 1SLSS ID or UE G-2: fAn IDin in_net is reserved for GNSS based synchronizationCoverage indicatorfor UE G-2: 0Note: The ID reserved for GNSS based synchronization hashigher priority than other IDs in id_net. option Priority: GNSS > UE G-1= UE N-1 > UE G-2 = 2 UE N-2 > OON UESLSS ID for UE G-1: in_net reservedfor GNSS based synchronizationCoverage indicator for UE G-1: 1SLSS IDfor UE G-2: in_net reserved for GNSS based synchronizationCoverageindicator for UE G-2: 0 option Priority: GNSS > UE G-1 > UE N-1 = 3 UEG-2 > UE N-2 > OON UESLSS ID for UE G-1: in_net reserved for GNSS basedsynchronizationCoverage indicator for UE G-1: 1New field using PSBCHreserved bits for indication of direct GNSS reception: 1SLSS ID for UEG-2: in_net reserved for GNSS based synchronizationCoverage indicatorfor UE G-2: 1New field using PSBCH reserved bits for indication ofdirect GNSS reception: 0 option Priority: UE N-1 > UE N-2 > GNSS > 4 UEG-1 > UE G-2 > OON UESLSS ID for UE G-1: an ID in id_oon is reserved forGNSS based synchronizationCoverage indicator for UE G-1: 1Or coverageindicator for UE G-1: 0 and new field for indication direct GNSSreception: 1SLSS ID for UE G-2: an ID in id_oon is reserved for GNSSbased synchronizationCoverage indicator for UE G-2: 0Or coverageindicator for UE G-1: 0 and new field for indication direct GNSSreception: 0 option Priority: GNSS > UE G-1 > UE G-2 > 5 UE N-1 > UEN-2 > OON UESLSS ID for UE G-1: an ID in id_net is reserved for GNSSbased synchronizationCoverage indicator for UE G-1: 1Or coverageindicator for UE G-1: 1 and new field for indication direct GNSSreception: 1SLSS ID for UE G-2: an ID in id_net is reserved for GNSSbased synchronizationCoverage indicator for UE G-2: 0Or coverageindicator for UE G-1: 1 and new field for indication direct GNSSreception: 0

Option 1 or Option 2 does not need a new PSBCH field, and may minimizethe influence on the legacy synchronization procedure. Accordingly,Option 1 or Option 2 may be supported for PC5-based V2V.

To cope with a high frequency offset, a high-density DMRS and acomb-type RS may be considered. A similar approach should be applied toa PSBCH for PCS-based V2V. For example, a PSBCH DMRS is located inSC-FDMA symbol 5 in the first slot and SC-FDMA symbol 1 in the secondslot. A new PSBCH format may be used in a high carrier frequency, andthe network may indicate which PSBCH format is used. That is, a PSBCHDMRS type, number, and position may be modified for the PCS-based V2Voperation.

Meanwhile, in order to maintain the legacy rel. 12/13 operations, eventhough a UE has received a GNSS signal, a different coverage indicatorand/or a different SLSS ID may be configured for the UE according to acoverage state. (Legacy operations are maintained). Herein, the eNB mayindicate, to the GNSS-receiving UE, a specific ID among SLSS IDsavailable for UEs receiving a GNSS signal directly. Owing to thisoperation, an in-coverage UE capable of GNSS reception can naturallyreceive a GNSS signal, having a higher priority than an out-of-coverageUE. Further, the GNSS-based UE naturally has a higher priority than theOON UE. For this operation, the eNB may signal a (pre-)configured SLSSID selected from SLSS id net to an in-coverage GNSS-receiving UE, andthe UE which has received a GNSS signal may transmit an SLSS, using theSLSS ID indicated by the eNB. Under circumstances, a configured SLSS IDand a pre-configured SLSS ID for the GNSS may be different. In the casewhere an out-of-coverage UE receives a GNSS signal, the coverageindicator may be set to 0 or an SLSS ID preset from SLSS ID_oon may beused. In this manner, SLSS transmission operations may be differentiatedfor in-coverage and out-of-coverage UEs receiving GNSS signals, and thein-coverage UE receiving a GNSS signal may have a higher priority.

This method reflects assignment of the same priority to a GNSS-basedSLSS and an SLSS of an in-coverage UE from the foregoing description.Herein, to maintain the legacy operations, an out-of-coverage UE has alower priority than an in-coverage GNSS-based UE and a higher prioritythan the OON UE, even though the out-of-coverage UE is a (direct)GNSS-based UE.

Meanwhile, if an SLSS is transmitted using a separate PSSS root index(e.g., 38) instead of an SLSS ID reserved for the GNSS in SLSS id_net inthe method of setting an incoverage indicator to 1 for a UE receiving aGNSS signal directly and setting the incoverage indicator to 0 for a UEnot receiving a GNSS signal directly, priority between the SLSS and anexisting SLSS may be preset or indicated by signaling of the eNB. Thepriorities of the SLSSs may be signaled on a PSBCH so that thepriorities may be transmitted even to out-of-coverage UEs.

Now, various embodiments of a synchronization method and procedure forenabling reliable communication between a V-UE and a P-UE will bedescribed below.

Method 1 to Method 3 are for a case where the P-UE does not turn on aGPS application.

Method 1—A case where a P-UE transmits an SLSS based on a cellulartiming or a GPS timing.

If the P-UE does not turn on a GPS application, the P-UE may determine atiming at which a D2D signal is to be transmitted, based on the latestturn-on time of the GPS application. For example, a UE which hasrecently turned on a GPS application and received a GPS signalsuccessfully may determine whether to transmit a D2D signal based on aGPS timing or a cellular timing according to the difference between thecorresponding time and a current time or the difference between the GPStiming and the cellular timing.

If the difference between the time of receiving the GPS signalsuccessfully and the current time is equal to or larger than apredetermined threshold, the D2D signal is transmitted based on thecellular timing, and if the difference is less than the threshold, theD2D signal is transmitted based on the GPS timing. The threshold may besignaled by the network or preset.

If a time of transmitting the D2D signal is apart from the receptiontime of the GPS signal by a predetermined threshold or more, the P-UEmay transmit an SLSS and the D2D signal based on the cellular timing,determining that the timing of the GPS signal is incorrect.

When receiving the GPS signal, the P-UE may calculate the differencebetween the timing of the GPS signal and the cellular timing. If thecalculated difference is larger than a predetermined threshold, the P-UEmay transmit the SLSS in pre-defined SLSS resources in a predeterminedperiod. The threshold may be signaled by the network or preset.

If the P-UE does not execute the GPS application within a predeterminedtime or does not have a GPS reception capability in an extreme case, theP-UE may always transmit a cellular timing-based SLSS, assuming that theP-UE has not received a GPS signal. Or the P-UE may be pre-configured totransmit a cellular timing-based SLSS all the time. If the P-UE operatesin a vehicle (V) mode, for example, the P-UE is connected to a vehicle,even the P-UE may follow the following SLSS transmission rule for aV-UE.

When the P-UE transmits the cellular timing-based SLSS, SLSS resourcesand an SLSS ID may be preset or configured by the network.

The SLSS transmission of the P-UE may be indicated by a physical-layersignal or a higher-layer signal by an eNB or an RSU, or only UEs havingthe RSRPs of signals received from the eNB or the RSU equal to or lessthan (less than) a predetermined threshold may selectively transmitSLSSs. The purpose of transmitting an SLSS by a P-UE is to allow anadjacent V-UE to perceive the existence of the P-UE. In this case, theP-UE may transmit the SLSS in a predetermined period even though theP-UE does not transmit data.

Method 2—A case where a P-UE intermittently wakes up and receives a GPSsignal.

If the P-UE does not turn on the GPS application or a predetermined timehas elapsed from the last time of turning on the GPS application, theP-UE may receive a GPS signal in a predetermined period by turning on aGPS circuit for the purpose of tracking the GPS timing. In this method,the P-UE basically transmits a D2D signal and an SLSS based on the GPStiming. To this end, the P-UE wakes up at every predetermined intervalor a time interval configured by the network and receives a GPS signal.Because a GPS signal is received just for the purpose of acquiring theGPS timing rather than for the purpose of location estimation, the GPStiming may be estimated by acquiring GPS time information included in anavigation message of one GPS signal even in an extreme case in whichonly the single GPS signal is received, without the need for searchingfor all of a plurality of GPS signals.

A GPS timing valid time for estimation of the GPS timing may be defined.The GPS timing valid time may be defined as a time after the last timeof turning on the GPS application or the last time of receiving timinginformation from a GPS. If this valid time exceeds a predeterminedthreshold, the P-UE may notify a higher layer of the need for receivinga GPS signal, or may estimate the GPS timing accurately byintermittently receiving a GPS signal before the valid time exceeds thepredetermined threshold. As the P-UE directly estimates a GPS timingintermittently in this method, a timing may be aligned between the P-UEand the V-UE.

If the P-UE estimates an accurate GPS timing, the P-UE may transmit aGPS timing-based SLSS and D2D signal. For example, the P-UE may transmitan SLSS in the same SLSS transmission rule as applied to the V-UE.

In the case where the P-UE is to receive an SLSS from the V-UE, SLSSresources and an SLSS ID transmitted by the V-UE may be signaled to theP-UE by a physical-layer signal or a higher-layer signal. The P-UE maysearch for the SLSS in the corresponding resources.

Method 3a—A case where a V-UE transmits an SLSS based on a GPS timing.

A method for transmitting an SLSS at a predetermined subframe offsetposition in a predetermined period to a P-UE which never turns on a GPScircuit or a P-UE having no GPS reception capability by a V-UE isproposed. The V-UE may transmit an SLSS at a predetermined resourceposition based on a GPS timing, and the network may signal a windowvalue indicating that the SLSS is transmitted within +/−w from an SLSStransmission offset in order to reduce the complexity of searching forthe SLSS transmitted by the V-UE in the P-UE. A serving cell or campingcell of the P-UE may signal the transmission period and offset of theSLSS transmitted by the V-UE with respect to SFN 0 of the serving cellor camping cell to the P-UE. If the eNB or RSU is also capable ofreceiving a GPS signal, the eNB or RSU may signal a small w value, thusreducing the complexity required for searching for the SLSS of the V-UEin the P-UE. On the contrary, if the eNB or RSU is not capable ofreceiving a GPS signal, the eNB or RSU may signal a larger w valuebecause the eNB or RSU may have only rough timing information.

All V-UEs may not transmit an SLSS. Rather, V-UEs capable oftransmitting an SLSS may be restricted to V-UEs having GPS measurementqualities equal to or higher than a predetermined threshold. This isdone to increase a timing accuracy by allowing only a UE having highreliability to transmit an SLSS. The threshold may be preset orconfigured by the network.

Or all V-UEs transmitting data may be allowed to transmit an SLSS. Thisis done to enable the P-UE to receive an SLSS and thus estimate moreaccurate synchronization irrespective of when the P-UE wakes up.

Or the network or RSU may indicate SLSS transmission to the V-UE by aphysical-layer signal or a higher-layer signal. Since there is apossibility of a P-UE existing around the RSU or eNB having knowledge ofan ambient environment or regional characteristics, the network or RSUindicates SLSS transmission to the V-UE. This method is advantageous inthat unnecessary transmission of an SLSS from a V-UE is prevented in anenvironment without a P-UE in the neighborhood.

Method 3b—A case where a V-UE transmits an SLSS/D2D signal based on acellular timing.

The V-UE may measure a GPS timing and a cellular timing. If thedifference between the GPS timing and the cellular timing is equal to orlarger than a predetermined threshold, the V-UE may transmit an SLSSbased on the cellular timing. In this method, the V-UE measures thecellular timing and if the cellular timing is different from the GPStiming by the predetermined threshold or more, transmits a cellulartiming-based D2D signal/SLSS to a P-UE. This method is advantageous inthat an additional complexity of receiving a GPS signal at the P-UE isnot increased. An SLSS resource and D2D resource pool transmitted by theV-UE may be signaled in advance to the P-UE. The period and offset ofSLSS resources may be signaled with respect to SFN 0 of a serving cell.Herein, the network may signal to the P-UE a window value w indicatingthat the SLSS is transmitted within +/−w from the SLSS transmissionoffset in order to reduce the search complexity of the P-UE.

All V-UEs may not transmit an SLSS. Rather, V-UEs capable oftransmitting an SLSS may be restricted to V-UEs having GPS measurementqualities equal to or higher than a predetermined threshold. This isdone to prevent unnecessary SLSS reception of P-UEs camped on the eNB,caused by SLSS transmission of a UE at a network boundary. The thresholdmay be preset or configured by the network.

Or all V-UEs transmitting data may be allowed to transmit an SLSS. Thisis done to enable the P-UE to receive an SLSS and thus estimate moreaccurate synchronization irrespective of when the P-UE wakes up.

Or the network or RSU may indicate SLSS transmission to the V-UE by aphysical-layer signal or a higher-layer signal. Since there is apossibility of a P-UE existing around the RSU or eNB having knowledge ofan ambient environment or regional characteristics, the network or RSUindicates SLSS transmission to the V-UE. This method is advantageous inthat unnecessary transmission of an SLSS from a V-UE is prevented in anenvironment without a P-UE in the neighborhood.

This method is applicable only to specific information such as an alarmmessage that the V-UE transmits for the P-UE. In general communicationbetween V-UEs, when a V-UE transmits a signal for a P-UE based on a GPStiming, the V-UE uses a timing for the P-UE, thereby facilitating theP-UE to detect/decode a signal of the V-UE.

Method 3c—A case where a V-UE transmits an SLSS based on a cellulartiming.

The V-UE may transmit only an SLSS at a cellular timing and thentransmit D2D data at a GPS timing. Herein, to indicate the GPS timing toa receiving UE, the V-UE may signal, to the P-UE, an offset valuebetween the GPS timing and the cellular timing, or the differencebetween the starting point of SFN0 set based on the GPS timing and thestarting point of SFN0 set based on the cellular timing by aphysical-layer signal or a higher-layer signal. For example, the offsetvalue between the V-UE and the GPS timing may be signaled by a PSBHCreserved bit. In another example, a channel in which the V-UE transmitsa kind of control information or some data information to the P-UE(e.g., a new PSCCH format) may be defined separately, and the offsetvalue between the cellular timing and the GPS timing may be signaled onthe channel. According to this method, the existence of the V-UE isindicated to the P-UE by transmitting an SLSS based on the cellulartiming, and then a timing offset for transmission of the V-UE istransmitted on a PSBCH or another sidelink channel so that the P-UE maydetermine a position at which the V-UE transmits a packet. This methodis advantageous in that a P-UE may receive a signal from a V-UE withoutincreasing the complexity of the P-UE. Or the V-UE may transmit an SLSSand an additional physical channel (e.g., PSCCH) at the cellular timingand directly signal a GPS timing value (e.g. by expressing the startingpoint of SFNO as a UTC time) on the physical channel transmitted at thesame timing as the SLSS.

Meanwhile, this method may not be for a P-UE only. This method may alsobe helpful to a V-UE which has failed to receive a GPS signal. The V-UEwhich has failed to receive a GPS signal may also receive an SLSS at thecellular timing, and receive GPS timing information (or timing offsetinformation) transmitted at the same timing as the SLSS, for use inlater reception of a packet.

Meanwhile, resources in which the V-UE transmits an SLSS may have thesame period as resources in which the P-UE receives an alarm message inMethods 3a, 3b, and 3c. This is done to enable the P-UE to efficientlyreceive an SS by transmitting the SS at a position where the P-UE alwayswakes up and receives a signal. Herein, the alarm message receptionperiod refers to a resource area in which the P-UE is configured toalways wake up and receive specific data transmitted by the V-UE, theRSU, or another P-UE.

Since an operation for transmitting a signal to the V-UE or receiving amessage from the V-UE takes place intermittently in an event-triggeredmanner from the viewpoint of the P-UE, searching for an SS of the V-UEall the time by the P-UE may be inefficient. Therefore, the P-UE wakesup and searches for an SLSS of the V-UE in predetermined resources, orwakes up intermittently and receives a GPS signal, thereby maintainingonly synchronization to enable immediate Tx/Rx. In this manner, the P-UEmay reduce its battery consumption.

Method 3d—An eNB or RSU transmits a PSS/SSS or an SLSS. Herein, the eNBor RSU, which is capable of acquiring GPS synchronization, may indicateto a P-UE or a V-UE which has lost a GPS signal whether the GPSsynchronization has been acquired by a physical-layer signal or ahigher-layer signal. In this case, the P-UE or V-UE which has notreceived a GPS signal may transmit and receive a D2D signal based on thetiming (cellular timing) of the PSS/SSS or SLSS transmitted by the eNBor RSU, without the need for additionally receiving a GPS signal.

Method 4—A case where a P-UE turns on a GPS application.

The P-UE may transmit and receive a D2D signal at a GPS timing in aresource pool signaled by a serving cell or camping cell. In this case,since the P-UE has knowledge of the accurate GPS timing, the P-UE mayadvantageously operate at the same timing as the V-UE.

Method 5 and Method 6 are especially for the out-of-coverage case.

Method 5—A V-UE transmits an SLSS for an out-of-coverage P-UE.

If a P-UE is out of coverage, the P-UE should basically search for anSLSS in all resources. In this case, an adjacent V-UE may also be out ofcoverage. If the V-UE is out of coverage, the V-UE may transmit an SLSSat a predetermined SLSS resource position based on a GPS timing, using arandomly selected SLSS ID or a predetermined SLSS ID.

The SLSS resource position, period, and SLSS ID of the SLSS transmittedby the V-UE may be preset V-UE-commonly. All V-UEs may transmit an SLSSor only a part of the V-UEs may transmit an SLSS. For example, only aV-UE which is to transmit data may transmit an SLSS. V-UEs allowed totransmit an SLSS may be restricted to a V-UE which has turned on orupdated the GNSS within a predefined threshold time or a V-UE having aGNSS measurement quality equal to or higher than a predetermined value.Or only a V-UE which has detected an SLSS transmitted by a P-UE maytransmit an SLSS. For this operation, the V-UE should search for an SLSSof a P-UE at a usual time.

Method 6—A P-UE transmits an SLSS for a V-UE.

If a P-UE has not turned on a GPS application during a time periodexceeding a predefined threshold or has a GPS measurement quality equalto or lower than a predetermined value, the P-UE may be configured totransmit an SLSS (having a UE-selected SLSS ID or a predetermined SLSSID) in predefined SLSS resources in a predetermined period. In the casewhere the P-UE selects an SLSS ID, a V-UE has no way to know the SLSS IDand thus should search all SLSS IDs. In the case the SLSS ID used by theP-UE is predetermined, the V-UE has only to detect the SLSS ID. Thismethod is intended to indicate the existence of an adjacent P-UE by anSLSS and thus prevent a V-UE from transmitting an SLSS unnecessarily.

Method 7—Differentiation between SLSSs transmitted at different timings.

A D2D UE transmitting a signal at a cellular timing and a D2D UEtransmitting a signal at a GPS timing may co-exist. Herein, SLSSstransmitted by the two different UEs need to be distinguished from eachother at the physical layer.

Method 7-1) Differentiation using PSSS and/or SSSS root sequence

An SLSS sequence used by a V-UE may be distinguished from an SLSSsequence used by a P-UE. For example, the V-UE may transmit an SLSSusing a PSSS root sequence ID unused by the P-UE. Herein, the rootsequence ID used by the P-UE may be one used for D2D in the legacy LTERelease 12/13, or may be a new root sequence ID which has not been usedfor a P-UE in the legacy D2D. A root sequence ID different from that ofthe P-UE and the root sequence ID used for the legacy D2D may beconfigured for the V-UE.

Meanwhile, a PSS for a legacy DL uses ZC sequences of root indexes 25,29, and 34, and a PSSS of a sidelink uses ZC sequences of root indexes26 and 37. Herein, it is proposed that a D2D UE transmitting a signalbased on a GPS timing uses a ZC sequence of root index 38. Since the ZCsequence of root index 38 is in a complex conjugate relationship withthe ZC sequence of PSS root index 25 used for the legacy DL, an existingDL ZC sequence receiver may be re-used. Further, if a UE transmitting anSLSS based on the GPS timing co-exists with a legacy D2D UE, the legacyUE does not detect a GPS-based SS, which enables the co-existence.

Method 7-2—Differentiation using a PSBCH

A timing at which an SLSS is transmitted or a UE which transmits theSLSS may be indicated in some PSBCH reserved bit. For example, thecoverage indicator may be set to different values for a UE transmittinga signal based on a cellular timing and a UE transmitting a signal basedon a GPS timing. Or different SLSS transmission timings may be indicatedby setting the PSBCH reserved bit to different values.

Method 7-3—Different periods/resource positions are set along with theabove two methods.

For example, a transmission period may be set to be longer than a legacytransmission period, 40 ms for a D2D UE using the GPS as a timingreference. Or an SLSS transmission period may be set to be differentaccording to a specific event or regional characteristics. For example,if an accident occurs between vehicles, the SLSS transmission period maybe increased. Or the transmission period may be increased in thepresence of an underground parking lot or tunnel in the neighborhood. Oralthough the same period is used, a part of 40-ms SLSS resources may bedistinguished as resources transmitted by a UE using the cellulartiming.

Although the above differentiation method may be implemented differentlyfor different SLSS transmitting timings (cellular timing or GPS timing),the differentiation method may also be implemented differently dependingon an SLSS transmission entity. For example, all or a part ofsequence/PSBCH reserved bit/SLSS resource period/SLSS transmissionresource position may be set to be different for SLSSs transmitted by aP-UE and a V-UE. Further, a part of the foregoing methods may be used asa method for distinguishing an SLSS transmitted by an RSU from an SLSStransmitted by any other UE.

Now, a description will be given of a method for establishing initialfrequency synchronization.

Method 8

Although time synchronization is established based on the GPS timing, itmay be preferable to acquire frequency synchronization using SLSSs orD2D RSs transmitted between two UEs. Herein, it is most preferable toacquire frequency synchronization for an initial D2D signal reception,using a PSS/SSS/CRS/CSI-RS/DMRS of a cellular network in the in-coveragecase. It is because unless frequency synchronization is established forthe initial D2D signal reception, the D2D signal reception performanceis degraded significantly. Accordingly, it is preferred that a UEbasically pre-acquires frequency synchronization using an RS (with arelatively small frequency offset) transmitted by a fixed node such asan eNB or RSU. For example, while a P-UE may set a timing based on anSLSS transmitted by a V-UE or using a GPS timing, the P-UE may acquireinitial frequency synchronization with the cellular network.

Method 9

A method for using frequency synchronization of a GPS signal may beconsidered for an out-of-coverage UE which cannot detect a signal fromthe network. A GPS receiver basically tracks frequency synchronizationby comparing a received pseudorandom noise (PRN) code with areceiver-generated PRN code. An oscillator may be corrected in a band inwhich D2D communication takes place, using the frequency synchronizationvalue. This method may be restrictively used only in the case where theD2D communication band is within a predetermined frequency spacing froma GPS band (e.g., 1.575 GHz or 1.227 GHz). Even though the GPS frequencysynchronization has been tracked accurately, if a carrier moves a lot,the accuracy of the GPS frequency synchronization may be decreased.Thus, a band in which GPS frequency synchronization is available may bepreset.

Method 9b

It may be regulated that a V-UE and/or a P-UE transmitting and receivinga V2X message should always transmit a message in frequencysynchronization with the GPS. In this method, a message is transmittedbased on the GPS frequency synchronization irrespective of coveragestates and the type of the message transmitted by a UE. The GPSfrequency synchronization is used even in a carrier in which a cellularsignal is transmitted. Considering that a UE is generally likely to haveone oscillator, if frequency synchronization is changed in transmittinga V2X message or a cellular message (Uu), this may increase theimplementation constraint of the UE due to the dynamic frequencyreference change. Thus, the GPS frequency synchronization is alwaysused. Further, as an eNB without a GPS receiver is configured to followthe GPS frequency synchronization, the frequency error between the GPSand the eNB may be reduced. Herein, the UE may signal a frequencysynchronization that the UE has used to the eNB by a physical-layersignal or a higher-layer signal. For example, when the V-UE sets the GPSas a frequency reference, the V-UE may notify the eNB that the GPS isset as a frequency reference by a PSBCH reserved bit or by setting anexisting field to a different value.

Method 9c

Meanwhile, if a UE which transmits a signal based on GPS frequencysynchronization fails to receive a GPS signal in method 9b, the UE mayuse eNB frequency synchronization as a fallback. If the UE fails toacquire the eNB frequency synchronization, the UE may acquire frequencysynchronization with a D2D signal transmitted by a UE which has receiveda GPS signal. If the UE also fails to acquire the frequencysynchronization with the D2D signal transmitted by the GPS-receiving UE,the UE may use its local oscillator. Herein, to indicate a frequencysynchronization which the UE has acquired to another UE, the UE may seta different SLSS sequence ID, different SLSS resources, a different D2Dsignal transmission resource area, or some field of a PSBCH to adifferent value. Or a combination of the foregoing methods may fallwithin the scope of the present disclosure.

Method 10

It may occur that a UE out of network coverage cannot acquire accuratefrequency synchronization because it fails to receive a GPS signalaround it or there is no device transmitting a signal continuously, suchas an eNB. In this case, a tighter frequency offset requirement than forother UEs may be set for the UE so that the frequency of the UE may notbe deviated by a predetermined threshold or above at an initialtransmission. Particularly, since a V-UE is highly likely to have a moreexpensive transceiver than a P-UE, a tighter frequency offsetrequirement may be set for the V-UE so that at least a frequency offsetgenerated in the V-UE may be reduced. Or if an RSU is a UE type, atighter frequency offset requirement than for other UEs may be set forthe RSU to thereby help adjacent UEs with frequency synchronization (ifthe RSU is an eNB type, the RSU will follow a frequency offsetrequirement for an eNB). A method for using additional SLSSresources/SLSS ID/sequence to indicate receiving UEs that a UE has atighter frequency offset requirement than for other UEs, or signalinginformation indicating the type/frequency requirement/frequencysynchronization priority of the UE on an additional physical channel(e.g., a PSBCH or any other channel) by the UE is proposed. Even thougha receiving UE detects a plurality of types of SLSSs/D2D signals, thereceiving UE is frequency-synchronized with a D2D signal of a UE havinga higher priority for corresponding information. Meanwhile, for a V-UE,mobility may increase and thus, it may be inappropriate to set frequencysynchronization as a reference. Therefore, it may be regulated that onlya fixed node such as an RSU, a V-UE having a speed equal to or lowerthan a predetermined value, a UE having a relative speed equal to orlower than (lower than) a predetermined speed, with respect to areceiving UE, or a UE having a tighter frequency offset requirement thanother UEs is set as a frequency synchronization reference, selectivelyor with priority. For this operation, for an SLSS transmitted by a UEsatisfying the foregoing condition, a different ID may be set,transmission resources may be separately configured, a different rootsequence may be used for a PSSS/or an SSSS, a PSBCH reserved bit may beset differently, or an indicator may be transmitted on an additionalphysical channel. For example, a V-UE moving at or below a predeterminedspeed may transmit information about its transmission speed on the PSBCHor another physical channel, and use a different SLSS ID according tothe speed.

Method 11

Frequency synchronization settings may be prioritized preliminarily. Forexample, frequency synchronizations may be prioritized preliminarily inthe order of eNB of cellular network >frequency synchronization of GPSsignal >V-UE or RSU (having a tighter frequency synchronizationrequirement than other UEs) >D2D signal transmitted by UE within networkcoverage >D2D signal transmitted by out-of-coverage UE, and it may beregulated that although a UE may receive a plurality of signals, the UEshould acquire frequency synchronization according to the presetpriority order. A part of the embodiments of frequency synchronizationprioritization may be excluded and the priority order of frequencysynchronizations may be changed. Further, the prioritization may beapplied to time synchronizations, not limited to frequencysynchronizations. Time synchronizations and frequency synchronizationsmay be prioritized in the same manner or in different manners. Forexample, the priorities of time synchronizations may be set in the orderof GPS >UE using GPS as reference, eNB, or RSU >signal transmitted by UEhaving mobility within predetermined threshold from among UEs using GPSas reference or UE-type RSUs >eNB asynchronous with GPS >UE withincoverage of eNB asynchronous with GPS >UE which has failed to receiveany of signals of GPS/eNB/eNB synchronized with eNB. The priority orderof time and/or frequency synchronizations may be pre-signaled by aphysical-layer signal or a higher-layer signal by the network.

Meanwhile, on the assumption that a frequency error of a UE synchronizedwith the GPS is within 0.05 ppm, the frequency error is lower than afrequency offset requirement 0.1 ppm for a UE synchronized with an eNB.Thus, it is preferable to synchronize with the GPS first of all. In thisaspect, the GPS has a highest priority, and a signal transmitted by a UEsynchronized with the GPS may have the next priority.

Meanwhile, an SLSS transmitted by a UE having a poor GPS signalreception performance may cause interference to another UE receiving theSLSS or a UE receiving a GPS signal normally. To avert this problem,Method 12 may be used.

Method 12—Transmission power of an SLSS is controlled according to a GPSmeasurement quality.

In Method 12, a UE having a good GPS measurement quality transmits anSLSS with higher power, and a UE having a poor GPS measurement qualitytransmits an SLSS with lower power. If a GPS measurement quality isequal to or lower than a predetermined threshold, an SLSS may not betransmitted in an extreme case. In a more specific embodiment, forexample, SLSS transmission power may be determined by i)P_SLSS=min(P_SLSS_max, P0+alpha*(GPS measurement quality)), ii)P_SLSS=P_SLSSTxpw*delta(GPS measurement quality>threshold), iii) settingtransmission power of P_SLSS in multiple steps, and determining thesteps according to a measurement quality. Herein, the measurementquality may be a GPS measurement quality, and P0 and alpha may beconstants preset or signaled by the network. The proposed method isintended to reflect a measurement quality in setting SLSS transmissionpower. This may be implemented in the form of controlling SLSStransmission power according to a synchronization error. For example, aUE having or expected to have a large synchronization error transmits anSLSS with low power, and a UE having or expected to have a smallsynchronization error transmits an SLSS with high power. For example, aUE receiving a GPS signal directly transmits an SLSS with high power,expecting a small synchronization error, whereas a UE that fails toreceive a GPS signal directly or is synchronized with an SS of an eNBtransmits an SLSS with low transmission power, expecting a largesynchronization error. More specifically, SLSS transmission power may bedetermined by Min(P0, Pmax-alpha*(measurement error)) being amodification to the above method. All or a part of P0, Pmax, and alphamay be preset or signaled by a physical-layer signal or a higher-layersignal by the network.

In another embodiment, SLSS transmission power may be determined in thefollowing manner.

P1 if measurement error <a1

P2 if a1 <=measurement error <a2

P3 if a2<=measurement error <a3

. . .

PN if aN-1<=measurement error <aN

Herein, parameters such as N, an, and Pn may be preset or signaled by aphysical-layer signal or a higher-layer signal by the network.

Meanwhile, a specific method for measuring a GPS measurementquality/error is proposed below.

The reception power of a preamble/known sequence among GPS signals maybe considered as a factor of a measurement quality. A hop count for GPSsynchronization may be considered as a factor of GPS measurement. Forexample, a hop count of 0 is considered for a UE receiving a GPS signaldirectly, a hop count of 1 is considered for a UE synchronized with anSLSS that an eNB or RSU transmits by receiving a GPS signal andsynchronizing with the GPS signal, and a hop count of 2 is consideredfor a UE synchronized with an SLSS transmitted by a UE synchronized withan SLSS. It may be assumed that as the hop count increases, a certainbias is imposed on a measurement, thus causing a larger measurementerror.

A time after GPS synchronization is missed may be reflected in ameasurement quality. An offset may be applied, determining that as atime increases after GPS synchronization is missed, the measurementquality decreases or a measurement error increases. The size of theoffset may be determined in proportion to the time or may be determinedto be a constant (discontinuous function) determined according to thelength of a GPS-lost time period.

A GPS measurement quality or error may be measured by all or a part ofthe above-described methods in combination, and SLSS transmission powermay be determined based on the measurement.

Meanwhile, the foregoing SLSS transmission power determination methodmay be used as a method for determining the transmission power of amessage transmitted by a V-UE. For example, if it is determined that asynchronization quality is low or a synchronization error is large, thetransmission power of a message is decreased, thus reducing interferencewith other UEs. Or if it is determined that a synchronization quality islow or a synchronization error is large (or if it is determined that themeasurement error exceeds a predetermined threshold), it may beregulated that the corresponding UE should transmit an SLSS and/or amessage in separate resources/resource area. This is done to enable thecorresponding V-UEs to transmit signals in separate resource areasbecause they are asynchronous with other V-UEs so that interference withor from the other V-UEs may be decreased.

The foregoing frequency synchronization establishment method may beextended as an SS selection method. That is, the proposed method isapplied to timing as well as frequency. The frequency synchronizationestablishment method may be used in conjunction with one of timesynchronization establishment methods.

PLMN and Selection of Synchronization Source

Hereinbelow, a description will be given of PLMN-related prioritizationin relation to the above-described prioritization of synchronizationsources.

FIG. 11 illustrates an example referred to for describing PLMN-relatedprioritization in V2V. As illustrated, it is assumed that UE A and UE Bconducting V2V communication with each other are located in a commonarea between PLMN A and PLMN B. It is also assumed that the GNSS haspriority over an eNB in PLMN A, and an eNB has priority over the GNSS inPLMN B. In this case, if UE A and UE B prioritize synchronizationsources to select a synchronization source, irrespective of PLMNs, aproblem may be produced. Specifically, in the case where even though UEA and UE B belong to PLMN A, UE A receives a PSBCH related to PLMN A andthus selects the GNSS having a higher synchronization source prioritylevel as a synchronization source, and UE B receives a PSBCH related toPLMN B and thus selects the GNSS having the higher synchronizationsource priority level as a synchronization source, UE A and UE B applydifferent priority levels to synchronization sources and also selectdifferent synchronization sources in spite of belonging to the samePLMN. In this case, UE A and UE B, which are synchronized to the GNSSand the eNB, respectively, have as much a synchronization difference asthe timing difference between the GNSS and the eNB. If two UEsconducting V2V communication with each other have a synchronizationdifference, a safety problem may be produced.

Therefore, to solve the above problem, a UE according to an embodimentof the present disclosure may receive a physical sidelink broadcastchannel (PSBCH), determine which one between the GNSS and an eNB is tobe selected as a synchronization source according to priorityinformation included in the PSBCH, and then receive an SLSS related tothe determined synchronization source. Herein, the UE may determine thatthe priority information is valid only if a public land mobile network(PLMN) related to the priority information is identical to a PLMN towhich the UE belongs. The PSBCH may include a PLMN ID. Further, the SLSSrelated to the synchronization source may be received directly from thesynchronization source or from the synchronization source throughanother UE which has received the SLSS.

The above configuration may prevent UE A and UE B from receiving PSBCHsrelated to different PLMNs, applying different priority levels tosynchronization sources, and thus selecting different synchronizationsources (the GNSS and the eNB) in FIG. 11. Since UE B has received thePSBCH related to PLMN B, UE B may determine that priority levelsindicated by the PSBCH are not valid and thus may not apply the prioritylevels. Then, UE B may receive the PSBCH related to PLMN A to which UE Bbelongs and apply priority levels indicated by the PSBCH in selecting asynchronization source, thereby acquiring the same synchronizationsource as UE A.

Method for Transmitting Synchronization Signal by UE, in the Case whereUE Selects GNSS as Synchronization Source

Now, a description will be given of transmission of a synchronizationsignal from a UE which has received an SLSS, in the case where the UEselects the GNSS as a synchronization source. In the followingdescription, selection of the GNSS as a synchronization source mayresult from, not limited to, prioritization based on match betweenPLMNs. That is, the following description may also apply to the case ofselecting the GNSS as a synchronization source, aside from theafore-described prioritization.

If the UE receives an SLSS for which the GNSS serves as asynchronization source, the UE may always transmit the SLSS. If UEsselect an eNB as a synchronization source, legacy LTE Release 12/13synchronization signal transmission requirements may be conformed to. Asthe GNSS is introduced as a new synchronization source, UEs synchronizedto the GNSS may transmit a synchronization signal to provide timinginformation to other UEs. UEs, which have secured predefined,sufficiently satisfactory synchronization reception capabilities, mayset the GNSS as a synchronization reference. These UEs may alwaystransmit an SLSS in predetermined resources or resources indicated by anetwork. Herein, if SLSS resources are not configured, thesynchronization signal transmission may be impossible.

If the GNSS is set as a synchronization reference, an SLSS ID may bepreset. It may be regulated that a UE which has received the SLSS IDdoes not relay the SLSS. This is because there is no need for separatelyconfiguring resources for relaying a GNSS-based synchronization signal,and on the assumption that if a UE receives a GNSS signal at least once,a corresponding clock may be maintained for long in most cases, there isno need for additional synchronization relaying.

If the UE receives the SLSS for which the GNSS servers as asynchronization source, the UE may transmit the SLSS for a predeterminedtime even though the UE loses GNSS reception. That is, it may beregulated that once a UE receives a GNSS signal successfully, the UEtransmits an SLSS for a predetermined time even though the UE loses GNSSreception. Although the UE loses GNSS reception, the UE may maintain acorresponding clock for a predetermined time. Therefore, the UE isallowed to transmit the SLSS at least for the predetermined time duringwhich the clock is maintained and thus help another UE which fails inGNSS reception to acquire synchronization. A timer length during whichthe SLSS may be transmitted in the case of GNSS reception failure may bepreset or signaled by a physical layer signal or a higher layer signalby the network.

Or a UE configured to have a better capability of maintaining a clockafter GNSS reception may transmit an SLSS for a longer timer. For thispurpose, it may be determined whether to transmit an SLSS according to aGNSS reception quality. This is to determine whether to transmit theSLSS on the basis of not a GLSS lost time but a timing error. This isadvantageous in that certain performance can be ensured even though UEsmaintain GNSS clocks for different time periods. The timing error usedto determine whether to transmit the SLSS may be preset or signaled by aphysical layer signal or a higher layer signal by the network.

In another method, among UEs which have succeeded in GNSS reception, aUE having an RSRP of a signal from an eNB equal to or larger than afirst threshold and less than a second threshold may be configured totransmit an SLSS with an ID reserved for the GNSS. Herein, the first orsecond threshold may be infinity. This implies that the network allowsUEs which have succeeded in GNSS reception and are located atpredetermined positions within network coverage to transmit a GNSS-basedsynchronization signal.

In another method, whether UEs which have succeeded in GNSS receptionare to transmit an SLSS may be indicated by a physical layer signal or ahigher layer signal by the network. The network may prevent unnecessarysynchronization signal transmission or protect a Uu operation bycontrolling/determining whether UEs within cell coverage are to transmitan SLSS. Or, on the contrary, the network may force UEs, which havesucceeded in GNSS reception, to transmit an SLSS, thereby making a V2Voperation smooth.

Meanwhile, the network may control SLSS/PSBCH transmission in coverage.If the eNB is configured to give priority to an eNB timing, an LTERelease 12/13 D2D mechanism may be reused. That is, it is networksignaling or an RSRP threshold, which triggers an SLSS/PSBCHtransmission. In the case where the eNB is configured to give priorityto a GNSS timing, if a UE has the GNSS timing with ‘sufficientreliability’, the UE may be configured to always transmit an SLSS/PSBCH,in order to minimally affect the legacy standard. The meaning of‘sufficient reliability’ may be discussed in RAN4. In the case ofout-coverage, the LTE Release 12/13 SLSS/PSBCH transmission requirementsmay be reused. Further, if a UE has a GNSS timing with sufficientreliability, the UE may always transmit an SLSS/PSBCH.

In summary, i) for an in-coverage UE, if the eNB gives priority to theeNB timing over the GNSS timing, the LTE Release 12/13 mechanism forSLSS/PSBCH transmission is reused. ii) For an in-coverage UE, in thecase where the eNB gives priority to the GNSS timing over the eNBtiming, if a UE has a GNSS timing with sufficient reliability, the UEalways transmits an SLSS/PSBCH,. iii) For an out-coverage UE, the LTERelease 12/13 SLSS/PSBCH transmission mechanism is reused. Further, ifthe UE has the GNSS timing with sufficient reliability, the UE alwaystransmits an SLSS/PSBCH.

In LTE Release 12/13, one synchronization resource is configured for anin-coverage UE, and two synchronization resources are configured for anout-coverage UE. In V2V, if UEs select the GNSS as a timing reference,one more synchronization resource may be additionally configured for thein-coverage UE. Although the GNSS may be regarded as a very wide cell,the GNSS is not capable of configuring synchronization resources. Thus,the eNB may configure SLSS resources on behalf of the GNSS.

When the eNB configures additional synchronization resources, a timingreference should be discussed. In LTE Release 12/13 D2D, all timingoffsets are configured with respect to SFN 0. If the eNB has a GNSSreception capability, a subframe number (SFN) is aligned with a D2Dframe number (DFN), and thus the eNB may configure an additionalsynchronization resource with respect to SFN 0. If the eNB does not havethe GNSS reception capability, an SLSS timing misalignment occursbetween cells. Since synchronization resources for the SLSS-based GNSSis for a GNSS cell, a reference timing should be relative to DFN 0.Particularly in an asynchronous scenario, it is favorable to have acommon SLSS transmission timing.

In summary, in the case where the eNB configures SLSS resources for aGNSS timing with respect to SFN 0, a timing misalignment may occurbetween cells in an asynchronous network. Therefore, i) twosynchronization resources may be configured for an in-coverage UE. Oneof the synchronization resources is for eNB-based SLSS/PSBCHtransmission, and the other is for GNSS-based SLSS/PSBCH transmission.ii) For the GNSS-based SLSS/PSBCH transmission for the in-coverage UE,the synchronization resources are configured with respect to DFN 0.

For an out-coverage UE, two resources may be configured with respect toDFN 0. For GNSS-based synchronization, one of the two resources may beconfigured for SLSS transmission for a UE synchronized directly to theGNSS, and the other resource may be configured for SLSS transmission fora UE synchronized indirectly to the GNSS. In the case where the UE doesnot select the GNSS as a timing reference, if the UE selects another UEas a synchronization reference, one of the two resources is used forSLSS transmission, and the other resource is used for SLSS tracking.

Meanwhile, unlike the above method, for in-coverage, the network mayalso configure two synchronization resources for the GNSS. One of thesynchronization resources is for a UE receiving the GNSS directly, andthe other synchronization resource is for SLSS transmission of a UEsynchronized to a UE which has received the GNSS. This scheme isintended to transmit and receive a GNSS-based timing stably by allowingGNSS relay even within the network.

In the case of out-coverage, one or two separate SLSS resources may alsobe (pre)configured for the GNSS. This means that the resources areadditionally allocated in order to prevent a probable resource collisionwith an independent synchronization source.

Method for Processing Resource Overlap Caused by DifferentSynchronization Sources

Meanwhile, the network may configure a source pool by setting asynchronization source as a reference for the pool. That is, asynchronization source may be set for each resource pool. Herein, thesynchronization reference of a specific resource pool may be the GNSS.If the GNSS is a reference, a corresponding resource pool bitmap may beinterpreted with respect to DFN 0.

If SFN 0 being the eNB timing and DFN 0 are different in timing,subframes may partially overlap. When the network sets DFNs, a subframeto be used for a sidelink may overlap partially with a D (downlink) or S(special) subframe among cellular subframes. In this case, it ispreferred to exclude the subframe overlapped with the correspondingsubframe from a D2D resource pool. In this case, whether to exclude asubframe from sidelink subframes may depend on an overlap degree. Inthis context, the following methods may be considered.

If among cellular subframes, a subframe unavailable for a sidelink(e.g., a DL or special subframe, a sidelink subframe configured based ona cellular timing, or a synchronization subframe) is overlapped at leastpartially, the whole corresponding subframe is excluded from sidelinktransmission. In this method, a DL subframe at the cellular timingoverlaps with two DFN-based UL subframes, the two UL subframes areexcluded from sidelink subframes.

If among cellular subframes, a subframe unavailable for a sidelink isoverlapped to a degree equal to or larger than a predeterminedthreshold, the corresponding whole subframe is excluded from sidelinktransmission. In this method, even though a subframe is overlappedpartially, the subframe is not always excluded. Rather, if a subframe isoverlapped to a degree equal to or larger than the predeterminedthreshold, the subframe is excluded. The overlap threshold may be setdifferently depending on whether a DFN-based subframe is overlapped atthe start or end. For example, if the end of a sidelink subframe isoverlapped, it does not matter whether an area corresponding to the lastsymbol of the sidelink subframe overlaps with a cellular subframebecause the last symbol of the sidelink subframes is not used. However,it may be configured that if a sidelink transmitting UE affects acellular subframe due to Tx/Rx switching, the corresponding area is notoverlapped. In addition, a propagation delay may be reflected in theoverall threshold according to a cell size. For example, even though theend of a sidelink subframe is overlapped by the length of an OFDM symbolwithout a Tx/Rx switching area, the sidelink subframe may be usedcontinuously. The above two operations are performed to protect D and Ssubframes, when UEs within a cell conduct sidelink communication withrespect to the GNSS.

A UE may acquire a subframe number/boundary/SFN boundary of an eNBwithin a cell, and DFNs based on this and the GNSS timing. The UE mayrecognize the difference between the two, and regard them as subframesunavailable as D and S subframes with respect to an SFN. Herein, if asubframe is partially overlapped in the above-described method, thecorresponding subframe may be regarded as an unavailable subframe. Aresource pool bitmap may be applied to the remaining subframes exceptfor a subframe carrying a synchronization signal. Herein, the UE mayrecognize the difference between the SFN and the DFN differentlyaccording to the position of the UE. In an environment in which cellsare large, each UE may recognize a different SFN area, and thus theposition of an excluded subframe may be different for each UE. To avoidthis phenomenon, the network may signal a timing offset between the SFNand the DFN by a physical layer signal or a higher layer signal. The SFNmay be based on the transmission time of the eNB, and may be determinedby an average SFN boundary of a UE within a cell. The UE may determinehow much a DFN subframe and an SFN subframe are overlapped based onsignaling indicating the difference between the SFN and the DFN, andthus commonly exclude a subframe. To provide this information to a UEoutside the network coverage, the UE may signal offset informationbetween the SFN and the DFN by a physical layer signal or a higher layersignal (e.g., some field of a PSBCH). If a UE within the cell sets theeNB timing as a synchronization reference, UEs using the DFN outside theeNB may not have accurate knowledge of information about a pool used byUEs within the eNB. If the pool information is signaled on a PSBCH, theUE may identify an SFN boundary with respect to a DFN. Herein, the UEmay determine which subframes are D and S subframes and asynchronization subframe by using a TDD configuration in the cell, andset a resource pool bitmap by excluding these subframes.

Meanwhile, a specific TDD configuration may be preset for out-coverageUEs. Herein, on the assumption that the TDD configuration is a virtualcell, a resource pool bit map from which excluding D and S subframes areexcluded may be applied. This may be an operation for protecting D and Sin the virtual cell, regarding the virtual cell as TDD used by a UE in anearby cell, if there is the UE in the nearby cell in a partial coveragesituation, a UE within a cell uses TDD, and a TDD configuration isaligned with the preset TDD configuration. Herein, to perfectly protectthe nearby TDD cell, if an offset between the SFN and the DFN is knownfrom a PSBCH of another UE, or the difference between the SFN and theDFN is known preliminarily, corresponding out-coverage UEs may perform asidelink subframe exclusion operation to protect D and S subframes whichare partially overlapped. That is, an out-coverage UE expects that thereis a nearby cellular UE, and thus preliminarily excludes subframesoverlapped (at least partially) with D and S subframes by using a presetTDD configuration and offset information which is preset or acquiredfrom another UE. Likewise, the out-coverage UE may also signal theoffset between the used SFN and the DFN by a physical layer signal or ahigher layer signal (e.g., some field (reserved) of a PSBCH) in order toindicate which offset has been used to a UE within the network coverage.

Meanwhile, UEs within the cell may accurately interpret a bitmap of aneighbor cell, only when the UEs are aware of the difference between anSFN of the neighbor cell and a DFN. For this purpose, the network maysignal, to UEs, the difference between the SFN and the DFN in neighborcells, or the difference between an SFN of a serving cell and an SFN ofa neighbor cell by a physical layer signal or a higher layer signal.Further, the network may signal a TDD configuration and asynchronization subframe of the neighbor cell. Thus, it may bedetermined how many sidelink subframes are excluded in the neighborcell, and the resource pool bitmap may be accurately applied.

Meanwhile, if the difference between the SFN and the DFN is equal to orlarger than a predetermined value, the network may set an in-coverage UEto always prioritize a specific synchronization source. For example, itmay be regulated that if the difference between the SFN and the DFN isequal to or larger than the predetermined value, the UE prioritizes aneNB timing over a GNSS timing.

UE Operation in the Case of Three Synchronization Resources

Table 6 below describes an operation of a UE, in the case of threesynchronization resources.

TABLE 6 Resource 1 UE (“InC synchro- resource” Resource 2 Resourcenization or “1st OoC (“2nd OoC 3 (“GNSS state: resource”) resource”)resource”) Notes UE is InC, SS from R12/13 sync to SS_net, solution eNBPSBCH UE is InC, Reserved PSBCH used sync to SLSS to protect GNSS ID,cell-edge PSBCH UEs. Note that PSBCH should be transmitted on the firstresource to avoid interference with OoC GNSS UEs PSBCH. UE is SS fromR12/13 OoC, syn- SS_ solution chronized netPSBCH to InC UE with with/InC_flag = without 0 GNSS UE is SS from SS_ R12/13 OoC, syn- oonPSBCHsolution chronized with to OoC InC_flag = 0 UE with SS_net with/ withoutGNSS UE is SS from SS_ R12/13 OoC, syn- oonPSBCH solution chronized withto OoC InC_flag = 0 UE with (The sync SS_oon resource is and with/selected without depending GNSS on the resource of the incoming syncsource) UE is SS from SS_ R12/13 isolated oonPSBCH solution and withwithout InC_flag = 0 GNSS (Random sync resource) UE is SS from SS_Reserved PSBCH only OoC, syn- oonPSBCH SLSS includes chronized with ID,DFN-related to GNSS InC_flag = 0 PSBCH fields. The (Random thirdresource sync is needed to resource) avoid interference to cell-edgeUEs.

TABLE 7 Resource 1 (“InC UE syn- resource” Resource 2 Resourcechronization or “1st OoC (“2nd OoC 3 (“GNSS state: resource”) resource”)resource”) Notes UE is InC, SS from R12/13 sync to eNB SS_net, solutionPSBCH UE is InC, Reserved PSBCH used sync to SLSS to protect GNSS ID,cell-edge PSBCH UEs. Note that PSBCH should be transmitted on the firstresource to avoid interference with OoC GNSS UEs PSBCH. UE is OoC, SSfrom R12/13 syn- SS_ solution chronized to netPSBCH InC UE with withoutInC_flag = GNSS 0 UE is OoC, SS from SS_ R12/13 syn- oonPSBCH solutionchronized to with OoC UE with InC_flag = 0 SS_net without GNSS UE isOoC, SS from SS_ R12/13 syn- oonPSBCH solution chronized to with OoC UEwith InC_flag = 0 SS_oon and (The sync without resource is GNSS selecteddepending on the resource of the incoming sync source) UE is SS from SS_R12/13 isolated oonPSBCH solution and with without InC_flag = 0 GNSS(Random sync resource) UE is OoC, SS from Reserved synchronized SS_ SLSSto GNSS if netPSBCH ID, incoverage with PSBCH UE's syn- InC_flag = (pre-chronization 0 (PSBCH configured signal is contents contents)successfully are aligned received. with in coverage UE's one) UE is OoC,Reserved synchronized SLSS to GNSS if ID, incoverage PSBCH UE's syn-(pre- chronization configured signal is not contents) successfullyreceived.

In the case where UE is OoC, synchronized to GNSS if incoverage UE'ssynchronization signal is successfully received in the above table, apart corresponding to Resource 3 (“GNSS resource”) may or may not betransmitted. If the part is not transmitted, a transmitting UE does nottransmit synchronization signals, thereby saving as much energy. If thepart is transmitted, the transmitting UE always transmits at least onesynchronization signal. Thus, a stable synchronization signal may betransmitted to a receiving UE.

Further, if a UE is synchronized to a synchronization signal insynchronization resource 3 (Resource 3), the following methods may beconsidered.

First, any of the remaining two resources may be randomly selected. Inthis method, UEs synchronized indirectly to a GNSS timing usedistributed resources. Secondly, to protect synchronization resources ofan in-coverage UE, a synchronization signal may be transmitted in asynchronization resource (Resource 2). This method may advantageouslyprotect in-coverage synchronization resources, and enable reception of asynchronization signal from an in-coverage UE all the time. Thirdly, thenetwork may configure or preconfigure resources to be used.

Meanwhile, UEs, which are synchronized to a synchronization signaltransmitted in Resource 2, may select one of the following methods inorder to select synchronization transmission resources: i) Resource 1 isused; ii) 1/3 of synchronization resources are randomly used; and iii)the network configures or preconfigures resources to be used.

Meanwhile, synchronization subframes are excluded from subframe indexingin current V2V. Therefore, even though TDD configurations are alignedbetween in-coverage and out-coverage, if the numbers of synchronizationresources are different, communication between in-coverage andout-coverage is impossible. Therefore, to avert this problem, it isproposed that a UE excludes the positions of synchronization resourcesset for out-coverage UEs from V2V subframes. For example, even thoughan-coverage UE transmits a synchronization signal only in onesynchronization resource, the UE does not perform V2V subframe indexingeither in the synchronization resources set for the out-coverage UEs.Then, an in-coverage UE and an out-coverage UE perform subframe indexingon the assumption of the same synchronization resources, therebyovercoming the foregoing problem.

Meanwhile, the network may configure two or three sidelink resources.Irrespective of the number of sidelink resources, the agreement onsynchronization source priority in the following Table 2 should besatisfied.

TABLE 8 If the UE detects no eNB in a carrier which is (pre-)configuredas the carrier which potentially includes eNBs used as sync reference,when the (pre)configuration information indicates that eNB timing hashigher priority than GNSS, the following priority rules should beapplied: -P1’: UE directly synchronized to eNB -P2’: UE indirectlysynchronized to eNB (i.e., UE whose SyncRef is another UE directlysynchronized to eNB) -P3’: GNSS -P4’: UE directly synchronized to GNSS-P5’: UE indirectly synchronized to GNSS (i.e., UE whose SyncRef isanother UE directly synchronized to GNSS) -P4’ and P5’ aredifferentiated at least when two sync resources are (pre)configured. FFSwhether P4’ and P5’ are differentiated when three sync resources are(pre)configured. -P6’: The remaining UEs have the lowest priority. Notethat when the (pre)configuration information indicates that GNSS hashigher priority than eNB timing, the following agreements are kept. -P1:GNSS -P2: the following UE has the same priority: -UE directlysynchronized to GNSS -UE directly synchronized to eNB -P3: the followingUE has the same priority: -UE indirectly synchronized to GNSS (if RAN1decides to differentiate between direct and indirect synchronization toGNSS) -UE indirectly synchronized to eNB -P4: the remaining UEs have thelowest priority.-When two resources are included, the following behavioris used. -The same sync resource is used for UEs directly synchronizedwith GNSS. -UE directly synchronized to GNSS is not required to monitorPSBCH in the other resource if GNSS is at the highest priority. -A UEthat selecting SLSS ID = 0 with in-coverage indicator = 1 as its syncreference transmits SLSS = 0 with in-coverage indicator = 0.-In-coverage indicator is used to differentiate direct GNSS andin-direct GNSS -UE directly and indirectly synchronized to GNSS setin-coverage indicator to 1 and 0 respectively. -SLSS ID 168 is used todifferentiate 1 hop sync. or more hops for GNSS based synchronization-FFS1 SLSS ID selection of “standalone UE”. -Note: The sync resource forthe in-coverage is one of the resource chosen from the out-of-coverageresources as D2D.-When three resources are included, the followingbehavior is used. -For UE InC following eNB timing: -Resource 1: PSBCHand SLSSID from NW, InC bit = 1. -For UE InC following GNSS timing:-Resource 1: PSBCH from NW, SLSSID = 0, InC bit = 1. -For UE OoC sync toUE InC: -Resource 2: PSBCH (except DFN) and SLSSID from Sync Ref, InCbit = 0. -For UE OoC sync to UE in partial coverage (i.e., with SLSSIDfrom net and InC = 0): -Resource 1: PSBCH (except DFN) from Sync Ref,SLSSID from Sync Ref + 168, InC bit = 0. -For UE OoC sync to UE OoC,distinguish 2 cases: -If sync Ref UE is directly sync to GNSS (i.e.,transmitting on resource 3). -Resource 2: PSBCH (except DFN) from SyncRef, InC bit = 0. FFS2 SLSS ID.-Other cases: -Use resource 1 or 2(different from Sync Ref) with PBSCH (except DFN) and SLSSID from SyncRef and InC bit = 0. -For UE isolated without GNSS: -Resource 1 or 2,randomly: PSBCH from preconfiguration, InC bit = 0. FFS3 SLSSIDselection -For UE OoC sync to GNSS. -Resource 3: PSBCH frompreconfiguration, SLSSID = 0, InC bit = 0. -FFS4 behavior if UE readsPSBCH from another UE. -Note that Resource 1 is “InC resource” or “1stOoC resource”, and Resource 2 and 3 are “2nd OoC resource” and “3rd OoCresource” respectively.

In order to distinguish indirectly synchronized to GNSS UE fromstandalone UE (P3 from P4 or P5′ from P6′), with the above prioritysatisfied, a specific SLSS ID may be reserved or a PSBCH field may beset differently, for the indirectly synchronized to GNSS UE. A case inwhich two resources are configured will be described. According to theagreement, SLSS ID=0 and incoverage indicator=1 for a UE directlysynchronized to the GNSS, and SLSS ID=0 and incoverage indicator=0 for aUE setting this UE as a synchronization reference. Therefore, if thestandalone UE (a UE which autonomously determines a synchronizationsignal timing) randomly selects one of SLSS IDs 168 to 335 as is doneconventionally, P3 or P5′ may be distinguished from P4 or P6′. That is,if two resources are configured, the standalone UE may select one ofSLSS IDs {168˜335}.

A case in which three synchronization resources are configured will beconsidered below.

If a UE sets the GNSS as a synchronization reference, the UE transmitsSLSS ID 0 in Resource 1 or 3. A synchronization signal relayed from thesignal may be transmitted in Resource 2. To distinguish asynchronization signal relayed from in-coverage from a synchronizationsignal relayed from out-coverage, it is proposed that one from SLSS IDoon is reserved for a synchronization signal relayed from in-coverage,and another from SLSS ID oon is reserved for a synchronization signalrelayed from out-coverage. For example, SLSS ID 168 is reserved for asynchronization signal propagated from in-coverage (and having aninitial synchronization derived from the GNSS), and SLSS ID 169 isreserved for transmission of a synchronization signal to relay a timing(of a UE synchronized SLSS ID 0 and is delivered in synch Resource 3)propagated from out-coverage (FFS3 in the above). In this case, it isproposed that the standalone UE selects one of SLSS IDs 170 to 335. Inthis method, a direct/indirect GNSS UE may be distinguished from and astandalone UE, and an SLSS transmitted from within the network coveragemay be distinguished from an SLSS transmitted from outside the networkcoverage. In a legacy Release 12/13 sidelink, a DMRS of a PSBCH isderived from an SLSS ID. In the case of same IDs and different PSBCHcontent, simultaneous reception of an SLSS and a PSBCH in the sameresources (preconfigured or configured by the network), decoding isimpossible because of the same DMRS but different codewords. To solvethe problem, a UE synchronized to SLSS ID 0 transmitted in Resource 3transmits SLSS ID 169 in Resource 2.

Meanwhile, in the case where the above proposal is accepted, if SLSSID=1 for an in-coverage UE, a UE relaying a synchronization signal tothe in-coverage UE in Resource 2 also uses the same SLSS ID (1+168),thus incurring a collision. Accordingly, it may be regulated that thenetwork does not use SLSS ID 1.

Meanwhile, according to the above proposal, the SLSS ID sets ofstandalone UEs are different in the case of two synchronizationresources and three synchronization resources. In the case of twosynchronization resources, SLSS IDs 168 to 335 may be selected, and inthe case of three synchronization resources, SLSS IDs 170 to 335 may beselected. Herein, in the case where different PLMNs are configured toperform sidelink transmission and reception in the same carrier, ifdifferent operators configure different numbers of synchronizationresources, an SLSS ID set selected by standalone UEs become obscure. Forexample, if operation A configures two synchronization resources andselects an SLS ID from among SLSS IDs 168 to 335, operator B configuresthree synchronization resources and selects an SLSS ID from among SLSSIDs 170 to 335,and a standalone UE selects SLSS ID 168, what priority isto be assigned to this SLSS ID becomes obscure. To eliminate theobscurity, it is proposed that a standalone UE selects an SLSS ID fromthe same SLSS ID set irrespective of the number of resources. That is,even though two synchronization resources are configured in the aboveproposal, one of SLSS IDs 170 to 335 is selected to eliminate obscurityin inter-PLMN sidelink communication. That is, according to theproposal, even though two synchronization resources are configured, SLSSIDs 168 and 169 are reserved without being used.

The above description is also applicable to a UL or a DL, not limited toD2D communication. Then, an eNB, a relay node, or the like may use theproposed methods.

Examples of the above-described proposed methods may be included as oneof methods for implementing the present disclosure. Thus, it is obviousthat the examples may be regarded as proposed methods. Further, whilethe proposed methods may be implemented independently, some proposedmethods may be combined (or integrated). It may be regulated thatinformation indicating whether the proposed methods are applied or not(or information about rules of the proposed methods) is transmitted to aUE in a predefined signal (e.g., a physical-layer signal or ahigher-layer signal) by an eNB.

Apparatuses according to Embodiment of the Disclosure

FIG. 11 illustrates configurations of a transmission point and a UEaccording to an embodiment of the present disclosure.

Referring to FIG. 11, a transmission point 10 according to the presentdisclosure may include a receiver 11, a transmitter 12, a processor 13,a memory 14, and a plurality of antennas 15. The plurality of antennas15 mean that the transmission point 10 supports MIMO transmission andreception. The receiver 11 may receive various UL signals, data, andinformation from a UE. The transmitter 12 may transmit various DLsignals, data, and information to a UE. The processor 13 may provideoverall control to the transmission point 10. According to an embodimentof the present disclosure, the processor 12 of the transmission point 10may process requirements of each of the foregoing embodiments.

Besides, the processor 13 of the transmission point 10 may compute andprocess information received by the transmission point 10 andinformation to be transmitted to the outside. The memory 14 may storecomputed and processed information for a predetermined time, and may bereplaced with a component such as a buffer (not shown).

With continued reference to FIG. 11, a UE 20 according to the presentdisclosure may include a receiver 21, a transmitter 22, a processor 23,a memory 24, and a plurality of antennas 25. The plurality of antennas25 mean that the UE 20 supports MIMO transmission and reception. Thereceiver 21 may receive various DL signals, data, and information froman eNB. The transmitter 22 may transmit various UL signals, data, andinformation to an eNB. The processor 23 may provide overall control tothe UE 20.

According to an embodiment of the present disclosure, the processor 23of the UE 20 may process requirements of each of the foregoingembodiments.

Besides, the processor 23 of the UE 20 may compute and processinformation received by the UE 20 and information to be transmitted tothe outside. The memory 24 may store computed and processed informationfor a predetermined time, and may be replaced with a component such as abuffer (not shown).

The aforementioned transmission point and UE may be implemented suchthat the above-described various embodiments of the present disclosureare independently applied or two or more thereof are simultaneouslyapplied, and description of redundant parts is omitted for clarity.

Further, the description of the transmission point 10 in the descriptionof FIG. 11 is applicable to a relay as a DL transmission entity and a ULreception entity, and the description of the UE 20 in the description ofFIG. 11 is applicable to a relay as a UL transmission entity and a DLreception entity.

The embodiments of the present disclosure may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof.

In a hardware configuration, the embodiments of the present disclosuremay be achieved by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

In a firmware or software configuration, a method according toembodiments of the present disclosure may be implemented in the form ofa module, a procedure, a function, etc. Software code may be stored in amemory unit and executed by a processor. The memory unit is located atthe interior or exterior of the processor and may transmit and receivedata to and from the processor via various known means.

As described before, a detailed description has been given of preferredembodiments of the present disclosure so that those skilled in the artmay implement and perform the present disclosure. While reference hasbeen made above to the preferred embodiments of the present disclosure,those skilled in the art will understand that various modifications andalterations may be made to the present disclosure within the scope ofthe present disclosure. For example, those skilled in the art may usethe components described in the foregoing embodiments in combination.The above embodiments are therefore to be construed in all aspects asillustrative and not restrictive. The scope of the disclosure should bedetermined by the appended claims and their legal equivalents, not bythe above description, and all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

Those skilled in the art will appreciate that the present disclosure maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent disclosure. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of thedisclosure should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentdisclosure or included as a new claim by a subsequent amendment afterthe application is filed.

Apparatuses according to Embodiment of the Disclosure

FIG. 11 illustrates configurations of a transmission point and a UEaccording to an embodiment of the present disclosure.

Referring to FIG. 11, a transmission point 10 according to the presentdisclosure may include a receiver 11, a transmitter 12, a processor 13,a memory 14, and a plurality of antennas 15. The plurality of antennas15 mean that the transmission point 10 supports MIMO transmission andreception. The receiver 11 may receive various UL signals, data, andinformation from a UE. The transmitter 12 may transmit various DLsignals, data, and information to a UE. The processor 13 may provideoverall control to the transmission point 10. According to an embodimentof the present disclosure, the processor 12 of the transmission point 10may process requirements of each of the foregoing embodiments.

Besides, the processor 13 of the transmission point 10 may compute andprocess information received by the transmission point 10 andinformation to be transmitted to the outside. The memory 14 may storecomputed and processed information for a predetermined time, and may bereplaced with a component such as a buffer (not shown).

With continued reference to FIG. 11, a UE 20 according to the presentdisclosure may include a receiver 21, a transmitter 22, a processor 23,a memory 24, and a plurality of antennas 25. The plurality of antennas25 mean that the UE 20 supports MIMO transmission and reception. Thereceiver 21 may receive various DL signals, data, and information froman eNB. The transmitter 22 may transmit various UL signals, data, andinformation to an eNB. The processor 23 may provide overall control tothe UE 20.

According to an embodiment of the present disclosure, the processor 23of the UE 20 may process requirements of each of the foregoingembodiments.

Besides, the processor 23 of the UE 20 may compute and processinformation received by the UE 20 and information to be transmitted tothe outside. The memory 24 may store computed and processed informationfor a predetermined time, and may be replaced with a component such as abuffer (not shown).

The aforementioned transmission point and UE may be implemented suchthat the above-described various embodiments of the present disclosureare independently applied or two or more thereof are simultaneouslyapplied, and description of redundant parts is omitted for clarity.

Further, the description of the transmission point 10 in the descriptionof FIG. 11 is applicable to a relay as a DL transmission entity and a ULreception entity, and the description of the UE 20 in the description ofFIG. 11 is applicable to a relay as a UL transmission entity and a DLreception entity.

The embodiments of the present disclosure may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof.

In a hardware configuration, the embodiments of the present disclosuremay be achieved by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

In a firmware or software configuration, a method according toembodiments of the present disclosure may be implemented in the form ofa module, a procedure, a function, etc. Software code may be stored in amemory unit and executed by a processor. The memory unit is located atthe interior or exterior of the processor and may transmit and receivedata to and from the processor via various known means.

As described before, a detailed description has been given of preferredembodiments of the present disclosure so that those skilled in the artmay implement and perform the present disclosure. While reference hasbeen made above to the preferred embodiments of the present disclosure,those skilled in the art will understand that various modifications andalterations may be made to the present disclosure within the scope ofthe present disclosure. For example, those skilled in the art may usethe components described in the foregoing embodiments in combination.The above embodiments are therefore to be construed in all aspects asillustrative and not restrictive. The scope of the disclosure should bedetermined by the appended claims and their legal equivalents, not bythe above description, and all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

Those skilled in the art will appreciate that the present disclosure maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent disclosure. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of thedisclosure should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentdisclosure or included as a new claim by a subsequent amendment afterthe application is filed.

The above embodiments of the present disclosure are applicable tovarious mobile communication systems.

What is claimed is:
 1. A method performed by a second user equipment(UE) in a wireless communication system, the method comprising;receiving, by the second UE from first UE directly synchronized to aGlobal Navigation Satellite System (GNSS), a first sidelinksynchronization signal (SLSS) with first SLSS ID, selecting, by thesecond UE, a second SLSS ID for a second SLSS based on the receivedfirst SLSS; and transmitting, by the second UE, the second SLSS,wherein, based on the first UE is out of a cell coverage and the firstSLSS is received on a first resource, a first value related toGNSS-synchronization propagation through the out of the cell coverage isselected as the second SLSS ID, and wherein, based on the first UE is inthe cell coverage and the first SLSS is received on a second resource, asecond value related to GNSS-synchronization propagation through thecell coverage is selected as the second SLSS ID.
 2. The method of claim1, wherein the first resource is different form the second resource. 3.A second User Equipment (UE) device comprising: at least one processor;and at least one computer memory operatively coupled to the at least oneprocessor and storing instructions which, when executed, cause the atleast one processor to perform operations, wherein the operationsinclude receiving, from first UE directly synchronized to a GlobalNavigation Satellite System (GNSS), a first sidelink synchronizationsignal (SLSS) with first SLSS ID, selecting a second SLSS ID for asecond SLSS based on the received first SLSS; and transmitting thesecond SLSS, wherein, based on the first UE is out of a cell coverageand the first SLSS is received on a first resource, a first valuerelated to GNSS-synchronization propagation through the out of the cellcoverage is selected as the second SLSS ID, and wherein, based on thefirst UE is in the cell coverage and the first SLSS is received on asecond resource, a second value related to GNSS-synchronizationpropagation through the cell coverage is selected as the second SLSS ID.4. The UE of claim 3, wherein the second UE is capable of communicatingwith at least one of another UE, a UE related to an autonomous drivingvehicle, the eNB, or a network.
 5. A processor in a wirelesscommunication system, the processor perform operations for a second userequipment (UE) wherein the operations include receiving, from first UEdirectly synchronized to a Global Navigation Satellite System (GNSS), afirst sidelink synchronization signal (SLSS) with first SLSS ID,selecting a second SLSS ID for a second SLSS based on the received firstSLSS; and transmitting the second SLSS, wherein, based on the first UEis out of a cell coverage and the first SLSS is received on a firstresource, a first value related to GNSS-synchronization propagationthrough the out of the cell coverage is selected as the second SLSS ID,and wherein, based on the first UE is in the cell coverage and the firstSLSS is received on a second resource, a second value related toGNSS-synchronization propagation through the cell coverage is selectedas the second SLSS ID.
 6. A non-transitory computer-readable storagemedium storing at least one computer program including instructionswhich, when executed by at least one processor, cause the at least oneprocessor to perform operations for a second user equipment (UE),wherein the operations include receiving, from first UE directlysynchronized to a Global Navigation Satellite System (GNSS), a firstsidelink synchronization signal (SLSS) with first SLSS ID, selecting asecond SLSS ID for a second SLSS based on the received first SLSS; andtransmitting the second SLSS, wherein, based on the first UE is out of acell coverage and the first SLSS is received on a first resource, afirst value related to GNSS-synchronization propagation through the outof the cell coverage is selected as the second SLSS ID, and wherein,based on the first UE is in the cell coverage and the first SLSS isreceived on a second resource, a second value related toGNSS-synchronization propagation through the cell coverage is selectedas the second SLSS ID.